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/widgets/more_tabs_button.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 import '../models/app_tab.dart';
+import '../providers/connectivity_controller.dart';
 import '../providers/pick_controller.dart';
 import '../providers/settings_controller.dart';
 import 'app_tab_view_builder.dart';
@@ -16,13 +17,16 @@ class MoreTabsButton extends StatelessWidget {
   Widget build(BuildContext context) {
     final pick = context.watch<PickController>();
     final settings = context.watch<SettingsController>();
+    final connectivity = context.watch<ConnectivityController>();
 
     // While picking (another app's GET_CONTENT request), MainShellView
     // already restricts the bottom nav to Files/Photos - this button would
     // otherwise be the one remaining way to reach a tab that isn't a valid
     // pick source (Trash, Shares, ...), so it hides entirely rather than
-    // just disabling.
-    if (pick.isPicking) return const SizedBox.shrink();
+    // just disabling. Same idea while offline: every hidden tab needs a
+    // network request to show anything useful, so there's nothing this
+    // button could usefully open.
+    if (pick.isPicking || connectivity.isOffline) return const SizedBox.shrink();
 
     return PopupMenuButton<AppTab>(
       icon: const Icon(Icons.apps_rounded),