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/models/app_tab.dart

@@ -4,7 +4,16 @@ import 'package:material_symbols_icons/symbols.dart';
 /// Every destination the bottom nav bar can show. Order here is only the
 /// fallback default — actual order/visibility/default-tab are user
 /// configurable and persisted in `SettingsController`.
-enum AppTab { files, photos, favorites, activity, trash, shares, recent }
+enum AppTab {
+  files,
+  photos,
+  favorites,
+  activity,
+  trash,
+  shares,
+  recent,
+  offline,
+}
 
 /// At most this many tabs may be visible in the bottom nav bar at once —
 /// the rest are reachable through the "more" dropdown instead.
@@ -27,6 +36,8 @@ extension AppTabInfo on AppTab {
         return 'Shares';
       case AppTab.recent:
         return 'Recent';
+      case AppTab.offline:
+        return 'Offline';
     }
   }
 
@@ -46,6 +57,8 @@ extension AppTabInfo on AppTab {
         return Icons.groups_rounded;
       case AppTab.recent:
         return Symbols.search_activity_rounded;
+      case AppTab.offline:
+        return Icons.offline_pin_rounded;
     }
   }
 }