Offline tab as Files view, automatic background sync, sync notifications

- Offline tab is now FilesView(offline: true) over a FolderBrowser interface
  (FilesController / OfflineController) sharing controls, tiles, thumbnails
- Synced files follow the Files Cache rule (default: refresh every 15 min):
  per-account WorkManager jobs, in-app timer, resume and pull triggers;
  root-etag shortcut skips full walks when nothing changed
- Single sync at a time (shared lock); logging out stops that account's sync
- Sync safety: PROPFIND failures skip the path instead of deleting local files
- Mirror empty folders and remove deleted ones; drop synced paths deleted on
  the server; missing local files are re-downloaded
- Notifications: silent per-account sync notifications, "Background sync
  notifications" setting, audible upload/download completion
- Files refresh in place (no spinner flash); retry after network returns
- Fix sync badges not updating (shared native status stream), "Sync off" on
  launch (eager providers), Files Cache section moved under Device Sync

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

lib/providers/photos_controller.dart

@@ -76,18 +76,27 @@ class PhotosController extends ChangeNotifier {
   Future<void> _onAccountActivated() async {
     final id = session.activeAccountId;
     if (id != null) {
-      final prefs = await session.prefsFuture;
-      String k(String base) => session.accountStore.accountPrefKey(id, base);
-      _showFavoritesOnly =
-          prefs.getBool(k(_prefShowFavoritesOnlyPhotos)) ?? false;
-      _showHidden = prefs.getBool(k(_prefShowHiddenPhotos)) ?? false;
-      final sortFieldName = prefs.getString(k(_prefSortField));
-      _sortField = FileSortField.values.firstWhere(
-        (f) => f.name == sortFieldName,
-        orElse: () => FileSortField.name,
-      );
-      _sortAscending = prefs.getBool(k(_prefSortAscending)) ?? true;
-      notifyListeners();
+      // Wrapped so a single bad/mistyped stored pref value can't
+      // silently skip fetchAllMedia() below and leave Photos permanently
+      // empty on this activation with no error surfaced anywhere - see
+      // FilesController._onAccountActivated's identical guard for the
+      // real incident this mirrors.
+      try {
+        final prefs = await session.prefsFuture;
+        String k(String base) => session.accountStore.accountPrefKey(id, base);
+        _showFavoritesOnly =
+            prefs.getBool(k(_prefShowFavoritesOnlyPhotos)) ?? false;
+        _showHidden = prefs.getBool(k(_prefShowHiddenPhotos)) ?? false;
+        final sortFieldName = prefs.getString(k(_prefSortField));
+        _sortField = FileSortField.values.firstWhere(
+          (f) => f.name == sortFieldName,
+          orElse: () => FileSortField.name,
+        );
+        _sortAscending = prefs.getBool(k(_prefSortAscending)) ?? true;
+        notifyListeners();
+      } catch (e) {
+        debugPrint('[PhotosController] Account-activation prefs restore failed: $e');
+      }
     }
     unawaited(fetchAllMedia());
   }