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/settings/settings_screen.dart

@@ -4,6 +4,7 @@ import 'package:provider/provider.dart';
 import '../library/database.dart';
 import '../library/permissions.dart';
 import '../services/navidrome_service.dart';
+import '../services/offline_mode.dart';
 import 'library_stats.dart';
 
 /// Settings-Tab: Bibliotheks-Statistik, Berechtigungen, Navidrome, Über Melo.
@@ -68,6 +69,19 @@ class _SettingsScreenState extends State<SettingsScreen> {
                     onPressed: () => _zeigeNavidromeDialog(),
                   ),
           ),
+          ListTile(
+            leading: const Icon(Icons.wifi_off),
+            title: const Text('📴 Offline-Modus'),
+            subtitle: const Text('Nur gecachte Lieder abspielen'),
+            trailing: Consumer<OfflineMode>(
+              builder: (context, offlineMode, _) {
+                return Switch(
+                  value: offlineMode.enabled,
+                  onChanged: (value) => offlineMode.setEnabled(value),
+                );
+              },
+            ),
+          ),
           const Divider(height: 1),
           const _SectionLabel('Berechtigungen'),
           ListTile(