fix: 6 Claude-Audit-Fixes (3 CRIT + 3 HIGH)

CRITICAL:
- Logout: LoginScreen statt MeloHome pushen (home_screen.dart)
- Path Traversal: sanitizeDateiname() zentral + sync-loop abgesichert
- SecureStorage: flutter_secure_storage für Token+Passwort (auth_service, navidrome)

HIGH:
- Seek: PlayerService.seek() reload-frei (audio_handler)
- Play/Pause: Zielzustand statt Toggle für System-Controls
- X-User: Header entfernt, Server verlässt sich auf JWT

lib/screens/home_screen.dart

@@ -12,6 +12,7 @@ import '../widgets/song_tile.dart';
 import 'download_screen.dart';
 import 'cloud_screen.dart';
 import 'settings_screen.dart';
+import 'login_screen.dart';
 import '../services/cloud_service.dart';
 import '../services/auth_service.dart';
 import '../config/app_config.dart';
@@ -397,7 +398,7 @@ class _MeloHomeState extends State<MeloHome> {
         await AuthService().logout();
         if (mounted) {
           Navigator.of(context).pushReplacement(
-            MaterialPageRoute(builder: (_) => const MeloHome()),
+            MaterialPageRoute(builder: (_) => const LoginScreen()),
           );
         }
         break;