feat(phase2): Caching + Offline-Mode Infrastruktur (P0)

CACHING:
- CacheManager: Speichert Stream-Lieder lokal mit MD5-Hash-Namensgebung
- getCacheSize(): Berechne Cache-Größe
- clearCache(): Lösche alle gecachten Lieder
- streamAndCache(): Stream gleichzeitig abspielen + cachen

OFFLINE-MODE:
- OfflineMode-Service: Toggle Offline-Modus
- Persistiert in SharedPreferences
- ChangeNotifier für UI-Updates
- Flag: Nur gecachte Lieder spielen

Infrastructure für Phase 2 P0 ready
Tests: 70/70 ✅

lib/services/offline_mode.dart

@@ -0,0 +1,23 @@
+import 'package:flutter/foundation.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+class OfflineMode extends ChangeNotifier {
+  static const _key = 'offline_mode_enabled';
+  bool _enabled = false;
+
+  bool get enabled => _enabled;
+
+  Future<void> init() async {
+    final prefs = await SharedPreferences.getInstance();
+    _enabled = prefs.getBool(_key) ?? false;
+    notifyListeners();
+  }
+
+  Future<void> setEnabled(bool value) async {
+    _enabled = value;
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setBool(_key, value);
+    notifyListeners();
+    debugPrint('Offline-Mode: ${value ? "AN" : "AUS"}');
+  }
+}