feat: WebDAV sync + batch DnD + clean shutdown + center-modal UX bundle

- Sync (WebDAV, auto-merge): UUID + tombstones foundations (server +
  JS), THTTPClient bridge cmds (get/put/test), runSyncNow engine with
  pull/merge/push flow, Settings UI, pre-sync backup option. Test
  connection now treats 404 as OK (snapshot not yet created) and 401/
  403 as auth failure with dedicated toast.
- Batch drag-drop: cards + table rows carry checked-set ids (CSV) when
  dragged from an active selection; folder + trash drop handlers parse
  and apply in batch via new moveEntriesToFolder helper that preserves
  TOTP / custom_fields / kind in the full PUT payload.
- Clean shutdown: WM_QUERYENDSESSION / WM_ENDSESSION captured in the
  bridge message-only window; FormCloseQuery bypasses the tray-minimize
  intercept on system shutdown / restart / logoff so FireDAC closes the
  SQLite WAL cleanly instead of leaving -shm / -wal residue after a
  force-kill.
- Center-mode modal: blur+dim backdrop via body::before pseudo-element
  in editor-position=center, swallows clicks below the panel so the
  existing outside-click handlers reliably dismiss the slideover /
  settings panel.
- Batch bar state fixes: state.checked cleared before render in
  moveEntriesToFolder, emptyTrash, and per-card restoreEntry /
  permanentDelete / deleteEntry so the action bar disappears once the
  selection is fully processed.
- Save-then-discard duplicate fix: soState reset to null before
  openSlideOver re-opens the freshly saved entry, otherwise the dirty
  check fired on the soState.id=null → newId switch and a Cancel left
  the form in new-entry mode (second Save → POST duplicate).
- TEST_SYNC.md: end-to-end checklist for validating the WebDAV sync
  with 2 real instances.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

delphi-backend/Source/PM.Database.pas

@@ -314,6 +314,42 @@ begin
   // Drives the card/table label so notes-with-fields read as "Credit card"
   // instead of the generic "Encrypted note" placeholder.
   AddColumnIfMissing('vault_entries', 'template',           'TEXT');
+  // Stable identity that survives export/import + cross-device sync.
+  // SQLite `id` is autoincrement local-only — useless to match the same
+  // logical entry across two installs. Populate existing rows with a
+  // fresh UUID v4 below so the migration is non-destructive.
+  AddColumnIfMissing('vault_entries', 'uuid',               'TEXT');
+  FConn.ExecSQL(
+    'CREATE INDEX IF NOT EXISTS idx_entries_uuid ' +
+    '  ON vault_entries(uuid)');
+  // Backfill UUIDs for legacy rows that landed before the column existed.
+  // SQLite has no native uuid() — emit one via hex(randomblob) + manual
+  // dashes (RFC 4122 v4 = 8-4-4-4-12 hex, version nibble forced to 4,
+  // variant nibble high bits 10).
+  FConn.ExecSQL(
+    'UPDATE vault_entries SET uuid = ' +
+    '  lower(hex(randomblob(4))) || ''-'' || ' +
+    '  lower(hex(randomblob(2))) || ''-4'' || ' +
+    '  substr(lower(hex(randomblob(2))), 2) || ''-'' || ' +
+    '  substr(''89ab'', 1 + (abs(random()) % 4), 1) || ' +
+    '  substr(lower(hex(randomblob(2))), 2) || ''-'' || ' +
+    '  lower(hex(randomblob(6))) ' +
+    'WHERE uuid IS NULL OR uuid = ''''');
+  // Tombstones: every hard-delete inserts a row here so the sync engine
+  // can propagate deletes to other devices without leaving deleted
+  // entries to silently reappear at next pull.
+  FConn.ExecSQL(
+    'CREATE TABLE IF NOT EXISTS entry_tombstones (' +
+    '  id INTEGER PRIMARY KEY AUTOINCREMENT,' +
+    '  user_id INTEGER NOT NULL,' +
+    '  uuid TEXT NOT NULL,' +
+    '  deleted_at DATETIME DEFAULT CURRENT_TIMESTAMP,' +
+    '  FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE,' +
+    '  UNIQUE(user_id, uuid)' +
+    ')');
+  FConn.ExecSQL(
+    'CREATE INDEX IF NOT EXISTS idx_tombstones_user ' +
+    '  ON entry_tombstones(user_id, deleted_at DESC)');
   // Per-folder customisation. NULL = no override → JS uses the default
   // accent + i-folder symbol.
   AddColumnIfMissing('folders',       'color',             'TEXT');