refactor(js): extract sync module from app.js monofile (§3.1)

Second slice of the app.js split (after crypto). Moves the WebDAV sync
section to js/app.sync.js: transport (_webdavCall), buildSyncSnapshot,
applyRemoteSnapshot (merge + tombstone arbitration), runSyncNow, and the
sync settings UI.

- Byte-for-byte identical to the extracted block (verified before removal);
  no duplicate const; no top-level sync reference left in app.js.
- Load order: AFTER app.js (unlike crypto, which loads before) because this
  module has a top-level side effect — `Bridge.onWebdavResult = …` — that
  needs Bridge/state/api already declared. Rule documented in CLAUDE.md.
- index.html + BuildAssets whitelist + harness APP_PARTS updated; assets
  rebuilt to embed the new file.
- Safety net: the existing merge tests exercise applyRemoteSnapshot /
  buildSyncSnapshot from the extracted file and stay green (42/42).

app.js: 11936 → 11256 lines (crypto + sync now separate).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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() {