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>

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"