diff --git a/CLAUDE.md b/CLAUDE.md
index 620ea98..8fcb36e 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -31,11 +31,18 @@ global** dans le navigateur, donc les `const`/fonctions d'un fichier sont
 visibles des suivants exactement comme dans le monofichier. Ordre actuel :
 
 ```
-js/argon2.js  (IIFE, globalThis.NobleArgon2)
+js/argon2.js      (IIFE, globalThis.NobleArgon2)
 js/app.crypto.js  (KDF, verifier, encrypt/decrypt — extrait §3.1)
-js/app.js  (le reste)
+js/app.js         (le reste : state, Bridge, api, UI…)
+js/app.sync.js    (WebDAV + merge — extrait §3.1, APRÈS app.js car
+                   effet de bord top-level `Bridge.onWebdavResult = …`)
 ```
 
+Note ordre : un module **sans** exécution top-level (que des déclarations,
+comme `app.crypto.js`) peut se charger AVANT `app.js`. Un module **avec** un
+effet de bord top-level qui touche un global d'`app.js` (`Bridge`, `state`…)
+doit se charger APRÈS (`app.sync.js`).
+
 Règles pour extraire un nouveau module :
 - Il doit se charger **avant** ses consommateurs et **ne jamais redéclarer**
   un `const` d'un autre fichier (un `const` dupliqué entre deux classic
@@ -87,6 +94,7 @@ parsing CSV d'import, et l'arbitrage merge/tombstone de sync
 | Handlers REST | `delphi-backend/Handlers/PM.Handler.*.pas` |
 | Frontend principal (en cours de découpage §3.1) | `js/app.js` |
 | Crypto frontend (KDF, verifier, AES-GCM) — extrait §3.1 | `js/app.crypto.js` |
+| Sync frontend (WebDAV, snapshot, merge) — extrait §3.1 | `js/app.sync.js` |
 | Argon2id vendé (bundle `@noble/hashes`, IIFE) | `js/argon2.js` |
 | HTML racine | `index.html` |
 | Styles | `css/style.css` |
diff --git a/CODE_AUDIT.md b/CODE_AUDIT.md
index 61bf799..fca8bff 100644
--- a/CODE_AUDIT.md
+++ b/CODE_AUDIT.md
@@ -224,11 +224,14 @@ seulement au runtime).
 chargés dans l'ordre via `<script>` séparés — **pas de bundler ES/esbuild**
 (les classic scripts partagent l'environnement lexical global, donc zéro
 réécriture des call-sites, risque quasi nul vs conversion en modules ES).
-- ✅ `js/app.crypto.js` extrait (KDF, verifier, AES-GCM) — vérifié
-  byte-for-byte identique à l'original, 42 tests verts, pas de `const`
-  dupliqué. Pattern + règles documentés dans CLAUDE.md « Découpage frontend ».
-- Reste à extraire (grosses sections cohésives) : sync, slideover,
-  settings, quicksearch, autofill…
+- ✅ `js/app.crypto.js` extrait (KDF, verifier, AES-GCM) — byte-for-byte
+  identique, 42 tests verts, pas de `const` dupliqué. Chargé AVANT app.js.
+- ✅ `js/app.sync.js` extrait (WebDAV, snapshot, merge `applyRemoteSnapshot`)
+  — byte-for-byte identique, couvert par les tests merge, chargé APRÈS app.js
+  (effet de bord top-level `Bridge.onWebdavResult`). Pattern + règles d'ordre
+  documentés dans CLAUDE.md « Découpage frontend ».
+- Reste à extraire (grosses sections cohésives) : slideover, settings,
+  quicksearch, autofill, auto-backup…
 - ✅ `node --check` en pré-étape de `BuildAssets.ps1` : **déjà fait** (cf. §3.2).
 
 ### 3.2 🟡 Aucun test automatisé — **partiellement adressé (2026-07-04)**
diff --git a/delphi-backend/assets/BuildAssets.ps1 b/delphi-backend/assets/BuildAssets.ps1
index cfe7885..aadd59b 100644
--- a/delphi-backend/assets/BuildAssets.ps1
+++ b/delphi-backend/assets/BuildAssets.ps1
@@ -54,6 +54,7 @@ $patterns = @(
     'js\argon2.js',
     'js\app.crypto.js',
     'js\app.js',
+    'js\app.sync.js',
     'css\style.css'
 )
 
diff --git a/delphi-backend/assets/assets.inc b/delphi-backend/assets/assets.inc
index 5899de0..9875e95 100644
--- a/delphi-backend/assets/assets.inc
+++ b/delphi-backend/assets/assets.inc
@@ -1,10 +1,11 @@
 // Auto-generated by BuildAssets.ps1 - do not edit by hand.
 const
-  EMBEDDED_ASSET_COUNT = 5;
+  EMBEDDED_ASSET_COUNT = 6;
   EMBEDDED_ASSETS: array[0..EMBEDDED_ASSET_COUNT-1] of TEmbeddedAsset = (
     (UrlPath: '/index.html'; ResName: 'INDEX_HTML'),
     (UrlPath: '/js/argon2.js'; ResName: 'JS_ARGON2_JS'),
     (UrlPath: '/js/app.crypto.js'; ResName: 'JS_APP_CRYPTO_JS'),
     (UrlPath: '/js/app.js'; ResName: 'JS_APP_JS'),
+    (UrlPath: '/js/app.sync.js'; ResName: 'JS_APP_SYNC_JS'),
     (UrlPath: '/css/style.css'; ResName: 'CSS_STYLE_CSS')
   );
diff --git a/delphi-backend/assets/assets.rc b/delphi-backend/assets/assets.rc
index e080722..be440e3 100644
--- a/delphi-backend/assets/assets.rc
+++ b/delphi-backend/assets/assets.rc
@@ -5,4 +5,5 @@ INDEX_HTML RCDATA "Z:\\password-manager\\index.html"
 JS_ARGON2_JS RCDATA "Z:\\password-manager\\js\\argon2.js"
 JS_APP_CRYPTO_JS RCDATA "Z:\\password-manager\\js\\app.crypto.js"
 JS_APP_JS RCDATA "Z:\\password-manager\\js\\app.js"
+JS_APP_SYNC_JS RCDATA "Z:\\password-manager\\js\\app.sync.js"
 CSS_STYLE_CSS RCDATA "Z:\\password-manager\\css\\style.css"
diff --git a/delphi-backend/assets/assets.res b/delphi-backend/assets/assets.res
index 5306684..a04b0bc 100644
Binary files a/delphi-backend/assets/assets.res and b/delphi-backend/assets/assets.res differ
diff --git a/index.html b/index.html
index c7562e1..6351c02 100644
--- a/index.html
+++ b/index.html
@@ -1195,5 +1195,6 @@
 <script src="js/argon2.js"></script>
 <script src="js/app.crypto.js"></script>
 <script src="js/app.js"></script>
+<script src="js/app.sync.js"></script>
 </body>
 </html>
diff --git a/js/app.js b/js/app.js
index 264a79a..b850e34 100644
--- a/js/app.js
+++ b/js/app.js
@@ -9768,598 +9768,10 @@ async function autoPurgeTrashIfNeeded() {
 }
 
 // ============================================================
-// SYNC (WebDAV, auto-merge with timestamps)
+// SYNC (WebDAV) — extracted to js/app.sync.js (§3.1), loaded as a
+// separate <script> AFTER this file (it needs Bridge/state/api at
+// load time for its Bridge.onWebdavResult handler).
 // ============================================================
-// Multi-device sync via a remote WebDAV server (Nextcloud, ownCloud,
-// Apache mod_dav, any compatible). The remote file is an encrypted
-// JSON snapshot (same crypto container as the export, separate
-// password — the sync password is device-local DPAPI and the user must
-// configure the same one on each device they want to sync).
-//
-// Strategy: auto-merge with last-write-wins on per-entry updated_at,
-// tombstones for delete propagation. No conflict UI in v1 — silent
-// resolution because solo personal use rarely produces simultaneous
-// edits across devices. Toast reports added / updated / deleted counts.
-//
-// Sensitive actions (export, change master pw…) keep their own reauth
-// path. Sync only touches entries + folders + tombstones.
-
-const SYNC_PREFS = {
-    enabled:   'syncEnabled',
-    url:       'syncUrl',         // e.g. https://cloud.example.com/remote.php/dav/files/USER/PMServer/vault-sync.json
-    user:      'syncUser',
-    pwd:       'syncPwd',         // WebDAV password / app token
-    encPwd:    'syncEncPwd',      // secret for the encrypted JSON container
-    preBackup: 'syncPreBackup',   // 'on' / '' — write a local copy before each sync
-    last:      'syncLast',        // ISO timestamp of last successful run
-};
-
-let _webdavResolvers = {};
-Bridge.onWebdavResult = function(reqId, status, payload, etag) {
-    const r = _webdavResolvers[reqId];
-    if (!r) return;
-    delete _webdavResolvers[reqId];
-    r({ status: status | 0, payload: payload || '', etag: etag || '' });
-};
-// method: 'get'|'put'|'test'. opts.ifMatch → sent as If-Match on a put so
-// the server rejects (412) a write when the remote changed since our pull.
-function _webdavCall(method, url, user, pwd, dataB64, opts) {
-    opts = opts || {};
-    const reqId = 'dav_' + Date.now() + '_' + Math.random().toString(36).slice(2, 8);
-    const CHUNK = 1000000;
-    // Large PUT bodies (a big encrypted vault) can't ride in a single
-    // cmd:// URL — WebView2 caps it and the navigation blanks the page.
-    // Stream the base64 through the file/chunk transport, then commit via
-    // webdav/put-commit (which reads the accumulated buffer server-side).
-    if (method === 'put' && dataB64 && dataB64.length > CHUNK) {
-        return (async () => {
-            const ok = await _streamChunks(reqId, dataB64);
-            if (!ok) return { status: 0, payload: 'chunk transfer failed', etag: '' };
-            return await new Promise(resolve => {
-                _webdavResolvers[reqId] = resolve;
-                let q = 'cmd://webdav/put-commit'
-                      + '?reqId=' + encodeURIComponent(reqId)
-                      + '&url='   + encodeURIComponent(url)
-                      + '&user='  + encodeURIComponent(user || '')
-                      + '&pwd='   + encodeURIComponent(pwd  || '');
-                if (opts.ifMatch) q += '&ifmatch=' + encodeURIComponent(opts.ifMatch);
-                window.location.href = q;
-                setTimeout(() => {
-                    if (_webdavResolvers[reqId]) {
-                        delete _webdavResolvers[reqId];
-                        resolve({ status: 0, payload: 'timeout', etag: '' });
-                    }
-                }, 60000);
-            });
-        })();
-    }
-    return new Promise(resolve => {
-        _webdavResolvers[reqId] = resolve;
-        let q = 'cmd://webdav/' + method
-              + '?reqId=' + encodeURIComponent(reqId)
-              + '&url='   + encodeURIComponent(url)
-              + '&user='  + encodeURIComponent(user || '')
-              + '&pwd='   + encodeURIComponent(pwd  || '');
-        if (dataB64) q += '&data=' + encodeURIComponent(dataB64);
-        if (opts.ifMatch) q += '&ifmatch=' + encodeURIComponent(opts.ifMatch);
-        window.location.href = q;
-        setTimeout(() => {
-            if (_webdavResolvers[reqId]) {
-                delete _webdavResolvers[reqId];
-                resolve({ status: 0, payload: 'timeout', etag: '' });
-            }
-        }, 60000);
-    });
-}
-
-function refreshSyncUI(cfg) {
-    if (!cfg) return;
-    const cb  = document.getElementById('settingSyncEnabled');
-    const cfg2 = document.getElementById('syncConfig');
-    if (cb) cb.checked = !!cfg.enabled;
-    if (cfg2) cfg2.style.display = cfg.enabled ? '' : 'none';
-    const url  = document.getElementById('syncUrl');
-    const user = document.getElementById('syncUser');
-    const pwd  = document.getElementById('syncPwd');
-    const pre  = document.getElementById('syncPreBackup');
-    if (url)  url.value  = cfg.url  || '';
-    if (user) user.value = cfg.user || '';
-    if (pwd)  pwd.value  = cfg.pwd  || '';
-    if (pre)  pre.checked = !!cfg.preBackup;
-    const pwdStatus = document.getElementById('syncPwdStatus');
-    if (pwdStatus) pwdStatus.textContent = cfg.encPwd ? 'Set.' : 'Not set.';
-    const last = document.getElementById('syncLast');
-    if (last) last.textContent = cfg.last
-        ? ' · Last: ' + cfg.last.replace('T', ' ').slice(0, 16)
-        : '';
-}
-
-async function syncSetEncPwdFlow() {
-    let err = '';
-    let n = 0;
-    for (;;) {
-        const v = await promptDialog({
-            title:    'Set sync password',
-            message:  'Use the SAME password on every device that syncs with this remote. ' +
-                      'Stored DPAPI-protected on this device only — never transmitted.',
-            placeholder: 'At least 8 characters',
-            password: true,
-            okText:   'Save',
-            error:    err,
-        });
-        if (!v) return;
-        if (v.length >= 8) {
-            Bridge.setPref(SYNC_PREFS.encPwd, v);
-            const pwdStatus = document.getElementById('syncPwdStatus');
-            if (pwdStatus) pwdStatus.textContent = 'Set.';
-            toast('Sync password saved');
-            return;
-        }
-        n++;
-        if (n >= 5) return toast('Too many invalid attempts', 'error');
-        err = 'Use at least 8 characters (attempt ' + n + ' / 5).';
-    }
-}
-
-async function loadSyncConfig() {
-    if (!Bridge.active) return null;
-    const [enabled, url, user, pwd, encPwd, pre, last] = await Promise.all([
-        Bridge.getPref(SYNC_PREFS.enabled),
-        Bridge.getPref(SYNC_PREFS.url),
-        Bridge.getPref(SYNC_PREFS.user),
-        Bridge.getPref(SYNC_PREFS.pwd),
-        Bridge.getPref(SYNC_PREFS.encPwd),
-        Bridge.getPref(SYNC_PREFS.preBackup),
-        Bridge.getPref(SYNC_PREFS.last),
-    ]);
-    return {
-        enabled:   enabled === '1',
-        url:       url    || '',
-        user:      user   || '',
-        pwd:       pwd    || '',
-        encPwd:    encPwd || '',
-        preBackup: pre === '1',
-        last:      last   || '',
-    };
-}
-
-async function syncTestConnection() {
-    const cfg = await loadSyncConfig();
-    if (!cfg || !cfg.url) return toast('Set the WebDAV URL first', 'warning');
-    toast('Testing connection…');
-    const r = await _webdavCall('test', cfg.url, cfg.user, cfg.pwd);
-    if (r.status >= 200 && r.status < 400) {
-        toast('Connection OK (' + r.status + ')');
-    } else if (r.status === 404) {
-        // Server reachable, snapshot file just doesn't exist yet — normal
-        // before the first sync. Treat as success.
-        toast('Connection OK · snapshot not created yet');
-    } else if (r.status === 401 || r.status === 403) {
-        toast('Auth failed (' + r.status + ') — check user/password', 'error');
-    } else if (r.status === 0) {
-        toast('Network error: ' + (r.payload || 'unreachable'), 'error');
-    } else {
-        toast('Server returned ' + r.status, 'error');
-    }
-}
-
-// Build the snapshot payload that gets encrypted + pushed to the remote.
-// Includes entries (decrypted plaintext, then re-encrypted under the
-// sync key), folders metadata, and tombstones. Mirrors doExport's shape
-// so a sync snapshot is also importable via "Import vault".
-async function buildSyncSnapshot() {
-    const payload = {
-        version:     1,
-        snapshot_at: new Date().toISOString(),
-        username:    state.username,
-        folders:     (state.folders || [])
-                       .filter(f => f && f.name && f.name !== 'All')
-                       .map(f => ({ name: f.name, color: f.color || '', icon: f.icon || '' })),
-        entries:     [],
-        tombstones:  [],
-    };
-    // Progress feedback: buildSyncSnapshot dominates runtime (per-entry
-    // decrypt + attachment fetch). Report N/total as we go so the user
-    // doesn't think the app froze on large vaults.
-    const eligible = state.entries.filter(e => e && e.uuid);
-    const total = eligible.length;
-    let done = 0;
-    for (const e of eligible) {
-        done++;
-        if (total > 5 && (done === 1 || done % 5 === 0 || done === total)) {
-            syncStatus('Preparing… ' + done + '/' + total);
-        }
-        const plain = await decryptPwd(e.encrypted_password, e.iv);
-        let plainTotp = '';
-        if (e.totp_secret && e.totp_iv) {
-            plainTotp = await decryptTotpSecret(e.totp_secret, e.totp_iv);
-            if (plainTotp === '[ERROR]') plainTotp = '';
-        }
-        let plainCustom = [];
-        if (e.custom_fields && e.custom_fields_iv) {
-            try { plainCustom = await decryptCustomFields(
-                e.custom_fields, e.custom_fields_iv); }
-            catch (_) {}
-        }
-        let attachments = [];
-        try {
-            const metas = await api('/entries/' + e.id + '/attachments',
-                                    { headers: authHeaders() });
-            for (const m of (metas || [])) {
-                const full = await api('/attachments/' + m.id,
-                                       { headers: authHeaders() });
-                const bytes = await decryptBlobBytes(full.encrypted_blob, full.iv);
-                attachments.push({
-                    filename:    m.filename, mime: m.mime,
-                    size_bytes:  m.size_bytes,
-                    content_b64: bytesToBase64(bytes),
-                });
-            }
-        } catch (_) {}
-        payload.entries.push({
-            uuid:           e.uuid,
-            site:           e.site || '',
-            title:          e.title || '',
-            username:       e.username || '',
-            password:       plain === '[ERROR]' ? '' : plain,
-            folder:         e.folder || 'All',
-            tags:           parseTags(e.tags),
-            favorite:       !!e.favorite,
-            totp_secret:    plainTotp,
-            kind:           e.kind || 'login',
-            template:       e.template || '',
-            custom_fields:  plainCustom,
-            attachments,
-            icon_b64:       e.icon_b64 || '',
-            created_at:     e.created_at,
-            updated_at:     e.updated_at,
-        });
-    }
-    try {
-        const ts = await api('/entries/tombstones', { headers: authHeaders() });
-        payload.tombstones = (Array.isArray(ts) ? ts : []).map(t => ({
-            uuid: t.uuid, deleted_at: t.deleted_at,
-        }));
-    } catch (_) {}
-    return payload;
-}
-
-async function applyRemoteSnapshot(remote) {
-    if (!remote || !Array.isArray(remote.entries)) return { added:0, updated:0, deleted:0, failed:0 };
-    let added = 0, updated = 0, deleted = 0, failed = 0;
-
-    // Ensure state.entries reflects the live DB before we read updated_at
-    // for the resurrection arbitration below — a restore-then-sync must
-    // see the restored rows' fresh timestamps.
-    await loadEntries();
-
-    // Apply remote tombstones — but arbitrate against local resurrections.
-    // A remote tombstone says "this uuid was deleted at T". If the local
-    // entry with that uuid was updated AFTER T (e.g. restored from a
-    // backup since the deletion), the resurrection wins and the tombstone
-    // is skipped — otherwise a restore would be silently undone on the
-    // next sync. Entries the local side hasn't touched since T are
-    // deleted normally (standard delete propagation).
-    if (Array.isArray(remote.tombstones) && remote.tombstones.length > 0) {
-        // Snapshot local uuid → updated_at BEFORE any deletion.
-        const localTs = new Map();
-        for (const e of state.entries)
-            if (e.uuid) localTs.set(e.uuid, e.updated_at || '');
-
-        // A local entry beats the tombstone only if it exists AND is
-        // provably newer than deleted_at. Unparseable/missing timestamps
-        // favour KEEP (data-loss is worse than a stale entry the user can
-        // re-delete).
-        const isResurrected = (uuid, deletedAt) => {
-            if (!localTs.has(uuid)) return false;         // not local → apply
-            const up = Date.parse(String(localTs.get(uuid)).replace(' ', 'T'));
-            const del = Date.parse(String(deletedAt || '').replace(' ', 'T'));
-            if (isNaN(up)) return true;                    // can't tell → keep
-            if (isNaN(del)) return false;                  // no delete time → apply
-            return up > del;
-        };
-
-        const toApply = remote.tombstones
-            .filter(t => t.uuid && !isResurrected(t.uuid, t.deleted_at))
-            .map(t => t.uuid);
-
-        if (toApply.length > 0) {
-            const preLocal = new Set();
-            for (const e of state.entries) if (e.uuid) preLocal.add(e.uuid);
-            try {
-                await api('/entries/tombstones', {
-                    method: 'POST',
-                    headers: authHeaders({ 'Content-Type': 'application/json' }),
-                    body: JSON.stringify({ uuids: toApply }),
-                });
-                deleted = toApply.filter(u => preLocal.has(u)).length;
-            } catch (_) {}
-        }
-    }
-
-    // Refresh local view AFTER tombstones so the maps reflect the cull.
-    await loadEntries();
-    const byUuid = new Map();
-    for (const e of state.entries) if (e.uuid) byUuid.set(e.uuid, e);
-
-    // Local tombstones must veto ANY remote entry with a matching uuid —
-    // otherwise a perm-delete on this device gets undone by the next
-    // pull ("48 added" after wiping the vault, because the remote
-    // snapshot still carries the pre-delete state). The tombstones are
-    // pushed back to the remote at the next push, propagating the
-    // delete cleanly.
-    const localTombstones = new Set();
-    try {
-        const ts = await api('/entries/tombstones', { headers: authHeaders() });
-        (Array.isArray(ts) ? ts : []).forEach(t => t.uuid && localTombstones.add(t.uuid));
-    } catch (_) {}
-
-    // Folders — add missing ones with the remote's color/icon. Existing
-    // folders are left untouched (user's local customisation wins).
-    if (Array.isArray(remote.folders)) {
-        const localNames = new Set((state.folders || []).map(f => f.name));
-        for (const f of remote.folders) {
-            if (!f.name || localNames.has(f.name)) continue;
-            try {
-                await api('/folders', {
-                    method: 'POST',
-                    headers: authHeaders({ 'Content-Type': 'application/json' }),
-                    body: JSON.stringify({
-                        name: f.name, color: f.color || '', icon: f.icon || '',
-                    }),
-                });
-            } catch (_) {}
-        }
-        await loadFolders();
-    }
-
-    // Per-entry merge.
-    for (const r of remote.entries) {
-        if (!r.uuid) continue;
-        // Skip entries this device has already tombstoned — never
-        // resurrect a deleted entry.
-        if (localTombstones.has(r.uuid)) continue;
-        const local = byUuid.get(r.uuid);
-        if (!local) {
-            // New entry on remote — encrypt locally + POST keeping the uuid.
-            try {
-                const enc = await encryptImportEntry(Object.assign({}, r, { uuid: r.uuid }));
-                const created = await api('/entries', {
-                    method: 'POST',
-                    headers: authHeaders({ 'Content-Type': 'application/json' }),
-                    body: JSON.stringify(enc),
-                });
-                added++;
-                // Restore attachments for this new entry.
-                if (Array.isArray(r.attachments) && created && created.id) {
-                    for (const a of r.attachments) {
-                        try {
-                            const bytes = base64ToBytes(a.content_b64 || '');
-                            const blob = await encryptBlobBytes(bytes);
-                            await api('/entries/' + created.id + '/attachments', {
-                                method: 'POST',
-                                headers: authHeaders({ 'Content-Type': 'application/json' }),
-                                body: JSON.stringify({
-                                    filename:       a.filename,
-                                    mime:           a.mime || 'application/octet-stream',
-                                    encrypted_blob: blob.encrypted,
-                                    iv:             blob.iv,
-                                    size_bytes:     a.size_bytes || bytes.length,
-                                }),
-                            });
-                        } catch (_) {}
-                    }
-                }
-            } catch (_) { failed++; }
-        } else {
-            // Both sides have it — keep the newer one (lexical ISO sort).
-            const remoteWins = (r.updated_at || '') > (local.updated_at || '');
-            if (!remoteWins) continue;
-            try {
-                const enc = await encryptImportEntry(r);
-                // PUT keeps the existing id but accepts the encrypted blobs.
-                // template + uuid not touched here (server preserves columns
-                // when fields aren't in body — uuid is immutable anyway).
-                await api('/entries/' + local.id, {
-                    method: 'PUT',
-                    headers: authHeaders({ 'Content-Type': 'application/json' }),
-                    body: JSON.stringify(enc),
-                });
-                updated++;
-            } catch (_) { failed++; }
-        }
-    }
-    return { added, updated, deleted, failed };
-}
-
-// Update the inline sync-status label next to the button. Empty string
-// hides it (end of sync). Called from every phase so the user sees
-// where the runtime is spent (buildSyncSnapshot is by far the slowest,
-// hence the per-entry counter).
-function syncStatus(text) {
-    const el = document.getElementById('syncStatus');
-    if (el) {
-        if (text) { el.textContent = text; el.style.display = ''; }
-        else      { el.textContent = '';  el.style.display = 'none'; }
-    }
-    // Also drive the global busy overlay so a running sync blocks stray
-    // clicks (e.g. the auto-backup "Choose…" picker) and reads the same
-    // as the manual backup. Empty text = clear.
-    if (text) showBusy(text); else hideBusy();
-    // Lock the Sync/Test buttons while a run is in flight so the user
-    // can't double-click a second concurrent sync.
-    const active = !!text;
-    ['syncNowBtn', 'syncTestBtn'].forEach(id => {
-        const b = document.getElementById(id);
-        if (b) b.disabled = active;
-    });
-}
-
-async function runSyncNow(_attempt) {
-    _attempt = _attempt || 0;
-    const cfg = await loadSyncConfig();
-    if (!cfg) return toast('Bridge not available', 'error');
-    if (!cfg.url) return toast('Sync not configured', 'warning');
-    if (!cfg.encPwd) return toast('Set the sync password first', 'warning');
-    if (!state.cryptoKey) return toast('Vault is locked', 'warning');
-
-    if (_attempt === 0) toast('Syncing…');
-    let merged = { added: 0, updated: 0, deleted: 0 };
-
-    // Fail-fast connectivity: hit the remote FIRST so a dead server /
-    // wrong URL aborts before any heavy local work (the pre-sync backup
-    // and buildSyncSnapshot are expensive on large vaults — no point
-    // running them if we can't reach the server).
-    syncStatus('Connecting…');
-    let pullResp;
-    try {
-        pullResp = await _webdavCall('get', cfg.url, cfg.user, cfg.pwd);
-    } catch (e) {
-        syncStatus('');
-        return toast('Sync pull failed: ' + (e && e.message || e), 'error');
-    }
-    if (pullResp.status === 0) {
-        syncStatus('');
-        return toast('Network error: ' + (pullResp.payload || 'unreachable'), 'error');
-    }
-    if (!(pullResp.status === 404 ||
-          (pullResp.status >= 200 && pullResp.status < 300))) {
-        syncStatus('');
-        return toast('Pull failed: HTTP ' + pullResp.status, 'error');
-    }
-    // ETag of the version we just pulled — sent back as If-Match on the
-    // push so the server rejects our write if another device changed the
-    // file in between (optimistic concurrency, avoids lost updates).
-    const remoteEtag = pullResp.etag || '';
-
-    // Decrypt + cross-account guard BEFORE touching local state, so a
-    // wrong sync password or a foreign account aborts cleanly.
-    let remoteSnap = null;
-    if (pullResp.status !== 404 && pullResp.payload) {
-        try {
-            const jsonText = new TextDecoder().decode(base64ToBytes(pullResp.payload));
-            const container = JSON.parse(jsonText);
-            remoteSnap = await decryptExportContainer(container, cfg.encPwd);
-        } catch (e) {
-            syncStatus('');
-            return toast('Remote decrypt failed — wrong sync password?', 'error');
-        }
-        if (remoteSnap && remoteSnap.username && state.username &&
-            remoteSnap.username !== state.username) {
-            // Drop the busy overlay so this confirm (z-index below it)
-            // is visible; a later syncStatus() re-shows it if we continue.
-            hideBusy();
-            const ok = await confirmDialog({
-                title:   'Different account on remote',
-                message: 'The remote snapshot belongs to <b>' +
-                         (remoteSnap.username + '').replace(/[<>&]/g, '') +
-                         '</b>, but you are signed in as <b>' +
-                         (state.username + '').replace(/[<>&]/g, '') +
-                         '</b>. Merging would mix the two vaults. ' +
-                         'Use a distinct sync URL per account.',
-                okText:  'Merge anyway',
-                cancelText: 'Cancel',
-                danger:  true,
-            });
-            if (!ok) { syncStatus(''); return toast('Sync cancelled — account mismatch', 'warning'); }
-        }
-    }
-
-    // Server reachable + snapshot decrypted → NOW do the optional
-    // pre-sync backup (captures current local state before the merge
-    // mutates it). Best-effort; a failure here doesn't block the sync.
-    if (cfg.preBackup) {
-        try {
-            const ab = await loadAutoBackupConfig();
-            const dir = (ab && ab.dir) ? ab.dir : null;
-            if (dir) {
-                syncStatus('Local backup…');
-                const snap = await buildSyncSnapshot();
-                const container = await encryptExportPayload(snap, cfg.encPwd);
-                const ts = new Date().toISOString()
-                    .replace(/[-:]/g, '').replace('T', '-').slice(0, 15);
-                const path = dir.replace(/[\\/]+$/, '') + '\\vault-presync-' + ts + '.json';
-                await Bridge.writeFile(path, JSON.stringify(container, null, 2));
-            }
-        } catch (_) { /* best-effort */ }
-    }
-
-    // Apply the remote snapshot (404 → nothing to merge, first sync).
-    if (remoteSnap) {
-        syncStatus('Merging…');
-        try {
-            merged = await applyRemoteSnapshot(remoteSnap);
-        } catch (e) {
-            syncStatus('');
-            return toast('Apply failed: ' + (e && e.message || e), 'error');
-        }
-    }
-
-    // Guard against data loss: if we failed to import ONE or more remote
-    // entries locally (server rejected them, encryption failed, etc.),
-    // pushing the current local snapshot would silently overwrite the
-    // remote file with a shrunken dataset. Abort the push and surface a
-    // clear error so the user can investigate + retry.
-    if (merged.failed && merged.failed > 0) {
-        syncStatus('');
-        return toast('Sync aborted — ' + merged.failed +
-            ' remote entry(ies) failed to import locally. Push skipped to avoid overwriting remote data.',
-            'error');
-    }
-
-    // Push merged state back to the remote.
-    let pushedCount = 0;
-    try {
-        await loadEntries();  // pull latest after applying remote changes
-        const snap = await buildSyncSnapshot();
-        pushedCount = Array.isArray(snap.entries) ? snap.entries.length : 0;
-        syncStatus('Encrypting…');
-        const container = await encryptExportPayload(snap, cfg.encPwd);
-        const bodyBytes = new TextEncoder().encode(JSON.stringify(container, null, 2));
-        syncStatus('Pushing…');
-        const r = await _webdavCall('put', cfg.url, cfg.user, cfg.pwd,
-                                    bytesToBase64(bodyBytes),
-                                    { ifMatch: remoteEtag });
-        // 412 Precondition Failed = the remote changed since our pull
-        // (another device pushed). Re-run the whole pull→merge→push so we
-        // fold in their changes instead of clobbering them. Bounded to a
-        // few tries to avoid a livelock against a device syncing in a hot
-        // loop.
-        if (r.status === 412) {
-            syncStatus('');
-            if (_attempt < 3) {
-                toast('Remote changed — re-syncing…');
-                return await runSyncNow(_attempt + 1);
-            }
-            return toast('Sync gave up after repeated remote changes — try again', 'error');
-        }
-        if (!(r.status >= 200 && r.status < 300)) {
-            syncStatus('');
-            return toast('Push failed: HTTP ' + r.status, 'error');
-        }
-    } catch (e) {
-        syncStatus('');
-        return toast('Sync push failed: ' + (e && e.message || e), 'error');
-    }
-
-    syncStatus('');
-    const now = new Date().toISOString();
-    Bridge.setPref(SYNC_PREFS.last, now);
-    render();
-    // Bidirectional summary: the added/updated/deleted counts are what
-    // was pulled FROM the remote into this device; pushedCount is the
-    // total entries written back to the remote (so "0·0·0 · pushed 12"
-    // makes clear the vault is safely uploaded even when nothing new
-    // came down).
-    const summary =
-        'pulled ' + merged.added + ' new · ' +
-        merged.updated + ' updated · ' +
-        merged.deleted + ' deleted · ' +
-        'pushed ' + pushedCount + ' ' +
-        (pushedCount === 1 ? 'entry' : 'entries');
-    toast('Sync complete — ' + summary);
-}
-
 // ============================================================
 // AUTO-BACKUP (silent encrypted exports on a schedule)
 // ============================================================
diff --git a/js/app.sync.js b/js/app.sync.js
new file mode 100644
index 0000000..6770508
--- /dev/null
+++ b/js/app.sync.js
@@ -0,0 +1,604 @@
+// ============================================================
+// app.sync.js — SYNC module (extracted from app.js, §3.1)
+// ============================================================
+//
+// Loaded as a classic <script> AFTER js/app.js (it has top-level side
+// effects — `Bridge.onWebdavResult = …` — that need `Bridge`, `state`, and
+// the api/loadEntries helpers already declared by app.js). Classic scripts
+// share one global lexical environment, so these functions see app.js's
+// globals and vice-versa exactly as in the monofile. See CLAUDE.md
+// "Découpage frontend" for the load-order rules.
+//
+// ============================================================
+// SYNC (WebDAV, auto-merge with timestamps)
+// ============================================================
+// Multi-device sync via a remote WebDAV server (Nextcloud, ownCloud,
+// Apache mod_dav, any compatible). The remote file is an encrypted
+// JSON snapshot (same crypto container as the export, separate
+// password — the sync password is device-local DPAPI and the user must
+// configure the same one on each device they want to sync).
+//
+// Strategy: auto-merge with last-write-wins on per-entry updated_at,
+// tombstones for delete propagation. No conflict UI in v1 — silent
+// resolution because solo personal use rarely produces simultaneous
+// edits across devices. Toast reports added / updated / deleted counts.
+//
+// Sensitive actions (export, change master pw…) keep their own reauth
+// path. Sync only touches entries + folders + tombstones.
+
+const SYNC_PREFS = {
+    enabled:   'syncEnabled',
+    url:       'syncUrl',         // e.g. https://cloud.example.com/remote.php/dav/files/USER/PMServer/vault-sync.json
+    user:      'syncUser',
+    pwd:       'syncPwd',         // WebDAV password / app token
+    encPwd:    'syncEncPwd',      // secret for the encrypted JSON container
+    preBackup: 'syncPreBackup',   // 'on' / '' — write a local copy before each sync
+    last:      'syncLast',        // ISO timestamp of last successful run
+};
+
+let _webdavResolvers = {};
+Bridge.onWebdavResult = function(reqId, status, payload, etag) {
+    const r = _webdavResolvers[reqId];
+    if (!r) return;
+    delete _webdavResolvers[reqId];
+    r({ status: status | 0, payload: payload || '', etag: etag || '' });
+};
+// method: 'get'|'put'|'test'. opts.ifMatch → sent as If-Match on a put so
+// the server rejects (412) a write when the remote changed since our pull.
+function _webdavCall(method, url, user, pwd, dataB64, opts) {
+    opts = opts || {};
+    const reqId = 'dav_' + Date.now() + '_' + Math.random().toString(36).slice(2, 8);
+    const CHUNK = 1000000;
+    // Large PUT bodies (a big encrypted vault) can't ride in a single
+    // cmd:// URL — WebView2 caps it and the navigation blanks the page.
+    // Stream the base64 through the file/chunk transport, then commit via
+    // webdav/put-commit (which reads the accumulated buffer server-side).
+    if (method === 'put' && dataB64 && dataB64.length > CHUNK) {
+        return (async () => {
+            const ok = await _streamChunks(reqId, dataB64);
+            if (!ok) return { status: 0, payload: 'chunk transfer failed', etag: '' };
+            return await new Promise(resolve => {
+                _webdavResolvers[reqId] = resolve;
+                let q = 'cmd://webdav/put-commit'
+                      + '?reqId=' + encodeURIComponent(reqId)
+                      + '&url='   + encodeURIComponent(url)
+                      + '&user='  + encodeURIComponent(user || '')
+                      + '&pwd='   + encodeURIComponent(pwd  || '');
+                if (opts.ifMatch) q += '&ifmatch=' + encodeURIComponent(opts.ifMatch);
+                window.location.href = q;
+                setTimeout(() => {
+                    if (_webdavResolvers[reqId]) {
+                        delete _webdavResolvers[reqId];
+                        resolve({ status: 0, payload: 'timeout', etag: '' });
+                    }
+                }, 60000);
+            });
+        })();
+    }
+    return new Promise(resolve => {
+        _webdavResolvers[reqId] = resolve;
+        let q = 'cmd://webdav/' + method
+              + '?reqId=' + encodeURIComponent(reqId)
+              + '&url='   + encodeURIComponent(url)
+              + '&user='  + encodeURIComponent(user || '')
+              + '&pwd='   + encodeURIComponent(pwd  || '');
+        if (dataB64) q += '&data=' + encodeURIComponent(dataB64);
+        if (opts.ifMatch) q += '&ifmatch=' + encodeURIComponent(opts.ifMatch);
+        window.location.href = q;
+        setTimeout(() => {
+            if (_webdavResolvers[reqId]) {
+                delete _webdavResolvers[reqId];
+                resolve({ status: 0, payload: 'timeout', etag: '' });
+            }
+        }, 60000);
+    });
+}
+
+function refreshSyncUI(cfg) {
+    if (!cfg) return;
+    const cb  = document.getElementById('settingSyncEnabled');
+    const cfg2 = document.getElementById('syncConfig');
+    if (cb) cb.checked = !!cfg.enabled;
+    if (cfg2) cfg2.style.display = cfg.enabled ? '' : 'none';
+    const url  = document.getElementById('syncUrl');
+    const user = document.getElementById('syncUser');
+    const pwd  = document.getElementById('syncPwd');
+    const pre  = document.getElementById('syncPreBackup');
+    if (url)  url.value  = cfg.url  || '';
+    if (user) user.value = cfg.user || '';
+    if (pwd)  pwd.value  = cfg.pwd  || '';
+    if (pre)  pre.checked = !!cfg.preBackup;
+    const pwdStatus = document.getElementById('syncPwdStatus');
+    if (pwdStatus) pwdStatus.textContent = cfg.encPwd ? 'Set.' : 'Not set.';
+    const last = document.getElementById('syncLast');
+    if (last) last.textContent = cfg.last
+        ? ' · Last: ' + cfg.last.replace('T', ' ').slice(0, 16)
+        : '';
+}
+
+async function syncSetEncPwdFlow() {
+    let err = '';
+    let n = 0;
+    for (;;) {
+        const v = await promptDialog({
+            title:    'Set sync password',
+            message:  'Use the SAME password on every device that syncs with this remote. ' +
+                      'Stored DPAPI-protected on this device only — never transmitted.',
+            placeholder: 'At least 8 characters',
+            password: true,
+            okText:   'Save',
+            error:    err,
+        });
+        if (!v) return;
+        if (v.length >= 8) {
+            Bridge.setPref(SYNC_PREFS.encPwd, v);
+            const pwdStatus = document.getElementById('syncPwdStatus');
+            if (pwdStatus) pwdStatus.textContent = 'Set.';
+            toast('Sync password saved');
+            return;
+        }
+        n++;
+        if (n >= 5) return toast('Too many invalid attempts', 'error');
+        err = 'Use at least 8 characters (attempt ' + n + ' / 5).';
+    }
+}
+
+async function loadSyncConfig() {
+    if (!Bridge.active) return null;
+    const [enabled, url, user, pwd, encPwd, pre, last] = await Promise.all([
+        Bridge.getPref(SYNC_PREFS.enabled),
+        Bridge.getPref(SYNC_PREFS.url),
+        Bridge.getPref(SYNC_PREFS.user),
+        Bridge.getPref(SYNC_PREFS.pwd),
+        Bridge.getPref(SYNC_PREFS.encPwd),
+        Bridge.getPref(SYNC_PREFS.preBackup),
+        Bridge.getPref(SYNC_PREFS.last),
+    ]);
+    return {
+        enabled:   enabled === '1',
+        url:       url    || '',
+        user:      user   || '',
+        pwd:       pwd    || '',
+        encPwd:    encPwd || '',
+        preBackup: pre === '1',
+        last:      last   || '',
+    };
+}
+
+async function syncTestConnection() {
+    const cfg = await loadSyncConfig();
+    if (!cfg || !cfg.url) return toast('Set the WebDAV URL first', 'warning');
+    toast('Testing connection…');
+    const r = await _webdavCall('test', cfg.url, cfg.user, cfg.pwd);
+    if (r.status >= 200 && r.status < 400) {
+        toast('Connection OK (' + r.status + ')');
+    } else if (r.status === 404) {
+        // Server reachable, snapshot file just doesn't exist yet — normal
+        // before the first sync. Treat as success.
+        toast('Connection OK · snapshot not created yet');
+    } else if (r.status === 401 || r.status === 403) {
+        toast('Auth failed (' + r.status + ') — check user/password', 'error');
+    } else if (r.status === 0) {
+        toast('Network error: ' + (r.payload || 'unreachable'), 'error');
+    } else {
+        toast('Server returned ' + r.status, 'error');
+    }
+}
+
+// Build the snapshot payload that gets encrypted + pushed to the remote.
+// Includes entries (decrypted plaintext, then re-encrypted under the
+// sync key), folders metadata, and tombstones. Mirrors doExport's shape
+// so a sync snapshot is also importable via "Import vault".
+async function buildSyncSnapshot() {
+    const payload = {
+        version:     1,
+        snapshot_at: new Date().toISOString(),
+        username:    state.username,
+        folders:     (state.folders || [])
+                       .filter(f => f && f.name && f.name !== 'All')
+                       .map(f => ({ name: f.name, color: f.color || '', icon: f.icon || '' })),
+        entries:     [],
+        tombstones:  [],
+    };
+    // Progress feedback: buildSyncSnapshot dominates runtime (per-entry
+    // decrypt + attachment fetch). Report N/total as we go so the user
+    // doesn't think the app froze on large vaults.
+    const eligible = state.entries.filter(e => e && e.uuid);
+    const total = eligible.length;
+    let done = 0;
+    for (const e of eligible) {
+        done++;
+        if (total > 5 && (done === 1 || done % 5 === 0 || done === total)) {
+            syncStatus('Preparing… ' + done + '/' + total);
+        }
+        const plain = await decryptPwd(e.encrypted_password, e.iv);
+        let plainTotp = '';
+        if (e.totp_secret && e.totp_iv) {
+            plainTotp = await decryptTotpSecret(e.totp_secret, e.totp_iv);
+            if (plainTotp === '[ERROR]') plainTotp = '';
+        }
+        let plainCustom = [];
+        if (e.custom_fields && e.custom_fields_iv) {
+            try { plainCustom = await decryptCustomFields(
+                e.custom_fields, e.custom_fields_iv); }
+            catch (_) {}
+        }
+        let attachments = [];
+        try {
+            const metas = await api('/entries/' + e.id + '/attachments',
+                                    { headers: authHeaders() });
+            for (const m of (metas || [])) {
+                const full = await api('/attachments/' + m.id,
+                                       { headers: authHeaders() });
+                const bytes = await decryptBlobBytes(full.encrypted_blob, full.iv);
+                attachments.push({
+                    filename:    m.filename, mime: m.mime,
+                    size_bytes:  m.size_bytes,
+                    content_b64: bytesToBase64(bytes),
+                });
+            }
+        } catch (_) {}
+        payload.entries.push({
+            uuid:           e.uuid,
+            site:           e.site || '',
+            title:          e.title || '',
+            username:       e.username || '',
+            password:       plain === '[ERROR]' ? '' : plain,
+            folder:         e.folder || 'All',
+            tags:           parseTags(e.tags),
+            favorite:       !!e.favorite,
+            totp_secret:    plainTotp,
+            kind:           e.kind || 'login',
+            template:       e.template || '',
+            custom_fields:  plainCustom,
+            attachments,
+            icon_b64:       e.icon_b64 || '',
+            created_at:     e.created_at,
+            updated_at:     e.updated_at,
+        });
+    }
+    try {
+        const ts = await api('/entries/tombstones', { headers: authHeaders() });
+        payload.tombstones = (Array.isArray(ts) ? ts : []).map(t => ({
+            uuid: t.uuid, deleted_at: t.deleted_at,
+        }));
+    } catch (_) {}
+    return payload;
+}
+
+async function applyRemoteSnapshot(remote) {
+    if (!remote || !Array.isArray(remote.entries)) return { added:0, updated:0, deleted:0, failed:0 };
+    let added = 0, updated = 0, deleted = 0, failed = 0;
+
+    // Ensure state.entries reflects the live DB before we read updated_at
+    // for the resurrection arbitration below — a restore-then-sync must
+    // see the restored rows' fresh timestamps.
+    await loadEntries();
+
+    // Apply remote tombstones — but arbitrate against local resurrections.
+    // A remote tombstone says "this uuid was deleted at T". If the local
+    // entry with that uuid was updated AFTER T (e.g. restored from a
+    // backup since the deletion), the resurrection wins and the tombstone
+    // is skipped — otherwise a restore would be silently undone on the
+    // next sync. Entries the local side hasn't touched since T are
+    // deleted normally (standard delete propagation).
+    if (Array.isArray(remote.tombstones) && remote.tombstones.length > 0) {
+        // Snapshot local uuid → updated_at BEFORE any deletion.
+        const localTs = new Map();
+        for (const e of state.entries)
+            if (e.uuid) localTs.set(e.uuid, e.updated_at || '');
+
+        // A local entry beats the tombstone only if it exists AND is
+        // provably newer than deleted_at. Unparseable/missing timestamps
+        // favour KEEP (data-loss is worse than a stale entry the user can
+        // re-delete).
+        const isResurrected = (uuid, deletedAt) => {
+            if (!localTs.has(uuid)) return false;         // not local → apply
+            const up = Date.parse(String(localTs.get(uuid)).replace(' ', 'T'));
+            const del = Date.parse(String(deletedAt || '').replace(' ', 'T'));
+            if (isNaN(up)) return true;                    // can't tell → keep
+            if (isNaN(del)) return false;                  // no delete time → apply
+            return up > del;
+        };
+
+        const toApply = remote.tombstones
+            .filter(t => t.uuid && !isResurrected(t.uuid, t.deleted_at))
+            .map(t => t.uuid);
+
+        if (toApply.length > 0) {
+            const preLocal = new Set();
+            for (const e of state.entries) if (e.uuid) preLocal.add(e.uuid);
+            try {
+                await api('/entries/tombstones', {
+                    method: 'POST',
+                    headers: authHeaders({ 'Content-Type': 'application/json' }),
+                    body: JSON.stringify({ uuids: toApply }),
+                });
+                deleted = toApply.filter(u => preLocal.has(u)).length;
+            } catch (_) {}
+        }
+    }
+
+    // Refresh local view AFTER tombstones so the maps reflect the cull.
+    await loadEntries();
+    const byUuid = new Map();
+    for (const e of state.entries) if (e.uuid) byUuid.set(e.uuid, e);
+
+    // Local tombstones must veto ANY remote entry with a matching uuid —
+    // otherwise a perm-delete on this device gets undone by the next
+    // pull ("48 added" after wiping the vault, because the remote
+    // snapshot still carries the pre-delete state). The tombstones are
+    // pushed back to the remote at the next push, propagating the
+    // delete cleanly.
+    const localTombstones = new Set();
+    try {
+        const ts = await api('/entries/tombstones', { headers: authHeaders() });
+        (Array.isArray(ts) ? ts : []).forEach(t => t.uuid && localTombstones.add(t.uuid));
+    } catch (_) {}
+
+    // Folders — add missing ones with the remote's color/icon. Existing
+    // folders are left untouched (user's local customisation wins).
+    if (Array.isArray(remote.folders)) {
+        const localNames = new Set((state.folders || []).map(f => f.name));
+        for (const f of remote.folders) {
+            if (!f.name || localNames.has(f.name)) continue;
+            try {
+                await api('/folders', {
+                    method: 'POST',
+                    headers: authHeaders({ 'Content-Type': 'application/json' }),
+                    body: JSON.stringify({
+                        name: f.name, color: f.color || '', icon: f.icon || '',
+                    }),
+                });
+            } catch (_) {}
+        }
+        await loadFolders();
+    }
+
+    // Per-entry merge.
+    for (const r of remote.entries) {
+        if (!r.uuid) continue;
+        // Skip entries this device has already tombstoned — never
+        // resurrect a deleted entry.
+        if (localTombstones.has(r.uuid)) continue;
+        const local = byUuid.get(r.uuid);
+        if (!local) {
+            // New entry on remote — encrypt locally + POST keeping the uuid.
+            try {
+                const enc = await encryptImportEntry(Object.assign({}, r, { uuid: r.uuid }));
+                const created = await api('/entries', {
+                    method: 'POST',
+                    headers: authHeaders({ 'Content-Type': 'application/json' }),
+                    body: JSON.stringify(enc),
+                });
+                added++;
+                // Restore attachments for this new entry.
+                if (Array.isArray(r.attachments) && created && created.id) {
+                    for (const a of r.attachments) {
+                        try {
+                            const bytes = base64ToBytes(a.content_b64 || '');
+                            const blob = await encryptBlobBytes(bytes);
+                            await api('/entries/' + created.id + '/attachments', {
+                                method: 'POST',
+                                headers: authHeaders({ 'Content-Type': 'application/json' }),
+                                body: JSON.stringify({
+                                    filename:       a.filename,
+                                    mime:           a.mime || 'application/octet-stream',
+                                    encrypted_blob: blob.encrypted,
+                                    iv:             blob.iv,
+                                    size_bytes:     a.size_bytes || bytes.length,
+                                }),
+                            });
+                        } catch (_) {}
+                    }
+                }
+            } catch (_) { failed++; }
+        } else {
+            // Both sides have it — keep the newer one (lexical ISO sort).
+            const remoteWins = (r.updated_at || '') > (local.updated_at || '');
+            if (!remoteWins) continue;
+            try {
+                const enc = await encryptImportEntry(r);
+                // PUT keeps the existing id but accepts the encrypted blobs.
+                // template + uuid not touched here (server preserves columns
+                // when fields aren't in body — uuid is immutable anyway).
+                await api('/entries/' + local.id, {
+                    method: 'PUT',
+                    headers: authHeaders({ 'Content-Type': 'application/json' }),
+                    body: JSON.stringify(enc),
+                });
+                updated++;
+            } catch (_) { failed++; }
+        }
+    }
+    return { added, updated, deleted, failed };
+}
+
+// Update the inline sync-status label next to the button. Empty string
+// hides it (end of sync). Called from every phase so the user sees
+// where the runtime is spent (buildSyncSnapshot is by far the slowest,
+// hence the per-entry counter).
+function syncStatus(text) {
+    const el = document.getElementById('syncStatus');
+    if (el) {
+        if (text) { el.textContent = text; el.style.display = ''; }
+        else      { el.textContent = '';  el.style.display = 'none'; }
+    }
+    // Also drive the global busy overlay so a running sync blocks stray
+    // clicks (e.g. the auto-backup "Choose…" picker) and reads the same
+    // as the manual backup. Empty text = clear.
+    if (text) showBusy(text); else hideBusy();
+    // Lock the Sync/Test buttons while a run is in flight so the user
+    // can't double-click a second concurrent sync.
+    const active = !!text;
+    ['syncNowBtn', 'syncTestBtn'].forEach(id => {
+        const b = document.getElementById(id);
+        if (b) b.disabled = active;
+    });
+}
+
+async function runSyncNow(_attempt) {
+    _attempt = _attempt || 0;
+    const cfg = await loadSyncConfig();
+    if (!cfg) return toast('Bridge not available', 'error');
+    if (!cfg.url) return toast('Sync not configured', 'warning');
+    if (!cfg.encPwd) return toast('Set the sync password first', 'warning');
+    if (!state.cryptoKey) return toast('Vault is locked', 'warning');
+
+    if (_attempt === 0) toast('Syncing…');
+    let merged = { added: 0, updated: 0, deleted: 0 };
+
+    // Fail-fast connectivity: hit the remote FIRST so a dead server /
+    // wrong URL aborts before any heavy local work (the pre-sync backup
+    // and buildSyncSnapshot are expensive on large vaults — no point
+    // running them if we can't reach the server).
+    syncStatus('Connecting…');
+    let pullResp;
+    try {
+        pullResp = await _webdavCall('get', cfg.url, cfg.user, cfg.pwd);
+    } catch (e) {
+        syncStatus('');
+        return toast('Sync pull failed: ' + (e && e.message || e), 'error');
+    }
+    if (pullResp.status === 0) {
+        syncStatus('');
+        return toast('Network error: ' + (pullResp.payload || 'unreachable'), 'error');
+    }
+    if (!(pullResp.status === 404 ||
+          (pullResp.status >= 200 && pullResp.status < 300))) {
+        syncStatus('');
+        return toast('Pull failed: HTTP ' + pullResp.status, 'error');
+    }
+    // ETag of the version we just pulled — sent back as If-Match on the
+    // push so the server rejects our write if another device changed the
+    // file in between (optimistic concurrency, avoids lost updates).
+    const remoteEtag = pullResp.etag || '';
+
+    // Decrypt + cross-account guard BEFORE touching local state, so a
+    // wrong sync password or a foreign account aborts cleanly.
+    let remoteSnap = null;
+    if (pullResp.status !== 404 && pullResp.payload) {
+        try {
+            const jsonText = new TextDecoder().decode(base64ToBytes(pullResp.payload));
+            const container = JSON.parse(jsonText);
+            remoteSnap = await decryptExportContainer(container, cfg.encPwd);
+        } catch (e) {
+            syncStatus('');
+            return toast('Remote decrypt failed — wrong sync password?', 'error');
+        }
+        if (remoteSnap && remoteSnap.username && state.username &&
+            remoteSnap.username !== state.username) {
+            // Drop the busy overlay so this confirm (z-index below it)
+            // is visible; a later syncStatus() re-shows it if we continue.
+            hideBusy();
+            const ok = await confirmDialog({
+                title:   'Different account on remote',
+                message: 'The remote snapshot belongs to <b>' +
+                         (remoteSnap.username + '').replace(/[<>&]/g, '') +
+                         '</b>, but you are signed in as <b>' +
+                         (state.username + '').replace(/[<>&]/g, '') +
+                         '</b>. Merging would mix the two vaults. ' +
+                         'Use a distinct sync URL per account.',
+                okText:  'Merge anyway',
+                cancelText: 'Cancel',
+                danger:  true,
+            });
+            if (!ok) { syncStatus(''); return toast('Sync cancelled — account mismatch', 'warning'); }
+        }
+    }
+
+    // Server reachable + snapshot decrypted → NOW do the optional
+    // pre-sync backup (captures current local state before the merge
+    // mutates it). Best-effort; a failure here doesn't block the sync.
+    if (cfg.preBackup) {
+        try {
+            const ab = await loadAutoBackupConfig();
+            const dir = (ab && ab.dir) ? ab.dir : null;
+            if (dir) {
+                syncStatus('Local backup…');
+                const snap = await buildSyncSnapshot();
+                const container = await encryptExportPayload(snap, cfg.encPwd);
+                const ts = new Date().toISOString()
+                    .replace(/[-:]/g, '').replace('T', '-').slice(0, 15);
+                const path = dir.replace(/[\\/]+$/, '') + '\\vault-presync-' + ts + '.json';
+                await Bridge.writeFile(path, JSON.stringify(container, null, 2));
+            }
+        } catch (_) { /* best-effort */ }
+    }
+
+    // Apply the remote snapshot (404 → nothing to merge, first sync).
+    if (remoteSnap) {
+        syncStatus('Merging…');
+        try {
+            merged = await applyRemoteSnapshot(remoteSnap);
+        } catch (e) {
+            syncStatus('');
+            return toast('Apply failed: ' + (e && e.message || e), 'error');
+        }
+    }
+
+    // Guard against data loss: if we failed to import ONE or more remote
+    // entries locally (server rejected them, encryption failed, etc.),
+    // pushing the current local snapshot would silently overwrite the
+    // remote file with a shrunken dataset. Abort the push and surface a
+    // clear error so the user can investigate + retry.
+    if (merged.failed && merged.failed > 0) {
+        syncStatus('');
+        return toast('Sync aborted — ' + merged.failed +
+            ' remote entry(ies) failed to import locally. Push skipped to avoid overwriting remote data.',
+            'error');
+    }
+
+    // Push merged state back to the remote.
+    let pushedCount = 0;
+    try {
+        await loadEntries();  // pull latest after applying remote changes
+        const snap = await buildSyncSnapshot();
+        pushedCount = Array.isArray(snap.entries) ? snap.entries.length : 0;
+        syncStatus('Encrypting…');
+        const container = await encryptExportPayload(snap, cfg.encPwd);
+        const bodyBytes = new TextEncoder().encode(JSON.stringify(container, null, 2));
+        syncStatus('Pushing…');
+        const r = await _webdavCall('put', cfg.url, cfg.user, cfg.pwd,
+                                    bytesToBase64(bodyBytes),
+                                    { ifMatch: remoteEtag });
+        // 412 Precondition Failed = the remote changed since our pull
+        // (another device pushed). Re-run the whole pull→merge→push so we
+        // fold in their changes instead of clobbering them. Bounded to a
+        // few tries to avoid a livelock against a device syncing in a hot
+        // loop.
+        if (r.status === 412) {
+            syncStatus('');
+            if (_attempt < 3) {
+                toast('Remote changed — re-syncing…');
+                return await runSyncNow(_attempt + 1);
+            }
+            return toast('Sync gave up after repeated remote changes — try again', 'error');
+        }
+        if (!(r.status >= 200 && r.status < 300)) {
+            syncStatus('');
+            return toast('Push failed: HTTP ' + r.status, 'error');
+        }
+    } catch (e) {
+        syncStatus('');
+        return toast('Sync push failed: ' + (e && e.message || e), 'error');
+    }
+
+    syncStatus('');
+    const now = new Date().toISOString();
+    Bridge.setPref(SYNC_PREFS.last, now);
+    render();
+    // Bidirectional summary: the added/updated/deleted counts are what
+    // was pulled FROM the remote into this device; pushedCount is the
+    // total entries written back to the remote (so "0·0·0 · pushed 12"
+    // makes clear the vault is safely uploaded even when nothing new
+    // came down).
+    const summary =
+        'pulled ' + merged.added + ' new · ' +
+        merged.updated + ' updated · ' +
+        merged.deleted + ' deleted · ' +
+        'pushed ' + pushedCount + ' ' +
+        (pushedCount === 1 ? 'entry' : 'entries');
+    toast('Sync complete — ' + summary);
+}
+
diff --git a/js/tests/harness.js b/js/tests/harness.js
index 86c99c7..55ec846 100644
--- a/js/tests/harness.js
+++ b/js/tests/harness.js
@@ -30,7 +30,7 @@ const { webcrypto } = require('node:crypto');
 // top-level const/let across separate runInContext calls, so we CONCATENATE
 // the app.* parts (in <script> load order) into one script. argon2.js is a
 // self-contained IIFE and loads separately (see below).
-const APP_PARTS = ['app.crypto.js', 'app.js'].map(f => path.join(__dirname, '..', f));
+const APP_PARTS = ['app.crypto.js', 'app.js', 'app.sync.js'].map(f => path.join(__dirname, '..', f));
 
 // In-memory Storage stub (Web Storage API surface used by app.js).
 function makeStorage() {