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 ✅

test/settings/settings_screen_test.dart

@@ -4,12 +4,14 @@ import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:provider/provider.dart';
 import 'package:melo/library/database.dart';
+import 'package:melo/services/offline_mode.dart';
 import 'package:melo/settings/settings_screen.dart';
 
 Widget _wrap(MeloDb db, Widget child) {
   return MultiProvider(
     providers: [
       Provider<MeloDb>.value(value: db),
+      ChangeNotifierProvider<OfflineMode>.value(value: OfflineMode()),
     ],
     child: MaterialApp(home: child),
   );