feat(settings): Offline-Mode Toggle UI (Phase 2)

- Toggle in Settings unter 'Musikserver'-Sektion
- Label: '📴 Offline-Modus' + Beschreibung
- Switch: Ein/Aus Toggle
- OfflineMode ChangeNotifierProvider in main.dart
- Tests angepasst für Provider-Setup

Nutzer kann jetzt Offline-Mode an/aus schalten!
Tests: 70/70 ✅

lib/main.dart

@@ -11,6 +11,7 @@ import 'player/audio_handler.dart';
 import 'player/mini_player.dart';
 import 'player/now_playing_screen.dart';
 import 'playlists/playlists_screen.dart';
+import 'services/offline_mode.dart';
 import 'settings/settings_screen.dart';
 import 'shared/theme.dart';
 
@@ -18,12 +19,15 @@ late final MeloAudioHandler _handler;
 late final MeloDb _db;
 late final LibraryService _library;
 late final PlaylistService _playlists;
+late final OfflineMode _offlineMode;
 
 Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
   _db = MeloDb();
   _library = LibraryService(_db);
   _playlists = PlaylistService(_db);
+  _offlineMode = OfflineMode();
+  await _offlineMode.init();
   _handler = await AudioService.init(
     builder: () => MeloAudioHandler(db: _db),
     config: const AudioServiceConfig(
@@ -46,6 +50,7 @@ class MeloApp extends StatelessWidget {
         Provider<MeloDb>.value(value: _db),
         ChangeNotifierProvider<LibraryService>.value(value: _library),
         ChangeNotifierProvider<PlaylistService>.value(value: _playlists),
+        ChangeNotifierProvider<OfflineMode>.value(value: _offlineMode),
       ],
       child: MaterialApp(
         title: 'Melo',