Add device sync: mirror folders/files locally, background WorkManager engine

- New native sync engine (SyncEngine/SyncWorker/ConflictResolveWorker,
  WorkManager-based) that mirrors selected Nextcloud folders or files to
  app-private local storage, periodically and on-demand ("Sync now"),
  with never-auto-resolved conflict notifications.
- Single files (not just folders) can now be marked "Sync to device".
- Live sync status (syncing/synced/conflicts) pushes from native to Dart
  over a new EventChannel; the persistent header chip/panel now reflects
  device-sync status instead of the WebDAV-refresh loading state, with a
  cloud_off/cloud_sync/cloud_done/cloud_alert icon set and an expandable
  conflicts list with in-app "Keep local"/"Use server" resolution.
  Per-item cloud_done/sync badges show on Files tiles.
- Share/Download short-circuit to the local copy for already-synced
  files instead of a fresh network fetch.
- Settings gained a Device Sync section (per-folder list, "Sync
  everything", Wi-Fi-only toggle, manual "Sync now").
- Fixed two release-only bugs found via on-device testing: Android's
  HttpURLConnection silently rejects the PROPFIND method (switched to
  OkHttp), and R8 was stripping WorkManager's reflection-instantiated
  internals (broadened proguard-rules.pro to keep androidx.work.**
  wholesale rather than chasing individual classes).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.claude/context/architecture.md

@@ -54,10 +54,10 @@ There is exactly one `ChangeNotifier`: [`ServerProvider`](../../lib/providers/se
 - UI settings that persist across launches — split into **global** (theme
   mode, seed color, dynamic-color toggle, bottom-bar opacity/blur,
   tap-tab-to-scroll-top, seek bar style, tab order/visibility/default, swipe
-  actions, login lock — see below) and **per-account** (grid/list view,
-  favorites-only, show-hidden, storage scope, Photos sort, Files'
-  per-folder sort map, cache policy) — see `server.md` for exactly which is
-  which and why
+  actions, login lock, sync-on-cellular — see below) and **per-account**
+  (grid/list view, favorites-only, show-hidden, storage scope, Photos sort,
+  Files' per-folder sort map, cache policy, synced folders) — see
+  `server.md` for exactly which is which and why
 - **Login lock** (`loginLockEnabled`/`lockAccountSwitching`/
   `lockHiddenFiles`/`needsUnlock`): an app-wide PIN/biometric gate via
   `AppLockService` (a thin wrapper over `local_auth` — this app never