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/cache_manager.dart

@@ -0,0 +1,71 @@
+import 'dart:io';
+import 'package:crypto/crypto.dart';
+import 'package:flutter/foundation.dart';
+import 'package:path_provider/path_provider.dart';
+
+class CacheManager {
+  static const _cacheDir = 'melo_cache';
+  late Directory _cacheFolder;
+
+  Future<void> init() async {
+    final appDir = await getApplicationCacheDirectory();
+    _cacheFolder = Directory('${appDir.path}/$_cacheDir');
+    if (!await _cacheFolder.exists()) {
+      await _cacheFolder.create(recursive: true);
+    }
+  }
+
+  Future<int> getCacheSize() async {
+    int size = 0;
+    final files = _cacheFolder.listSync(recursive: true);
+    for (final file in files) {
+      if (file is File) {
+        size += await file.length();
+      }
+    }
+    return size;
+  }
+
+  Future<void> clearCache() async {
+    try {
+      if (await _cacheFolder.exists()) {
+        await _cacheFolder.delete(recursive: true);
+      }
+      await _cacheFolder.create(recursive: true);
+      debugPrint('Cache gelöscht');
+    } catch (e) {
+      debugPrint('Fehler beim Löschen des Cache: $e');
+    }
+  }
+
+  Future<File?> getCachedFile(String streamUrl) async {
+    try {
+      final hash = md5.convert(streamUrl.codeUnits).toString();
+      final file = File('${_cacheFolder.path}/$hash.mp3');
+      if (await file.exists()) {
+        return file;
+      }
+      return null;
+    } catch (e) {
+      debugPrint('Fehler beim Abrufen der Cache-Datei: $e');
+      return null;
+    }
+  }
+
+  Future<File> getCacheFile(String streamUrl) async {
+    final hash = md5.convert(streamUrl.codeUnits).toString();
+    return File('${_cacheFolder.path}/$hash.mp3');
+  }
+
+  Stream<File> streamAndCache(Stream<List<int>> sourceStream, String streamUrl) async* {
+    final cacheFile = await getCacheFile(streamUrl);
+    final sink = cacheFile.openWrite();
+
+    await for (final chunk in sourceStream) {
+      sink.add(chunk);
+      yield cacheFile;
+    }
+
+    await sink.close();
+  }
+}

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"}');
+  }
+}