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/services/navidrome_service.dart

@@ -5,8 +5,10 @@ import 'package:crypto/crypto.dart';
 import 'package:http/http.dart' as http;
 import 'package:path_provider/path_provider.dart';
 import 'package:shared_preferences/shared_preferences.dart';
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
 import '../models/song.dart';
 import '../database/db_helper.dart';
+import '../utils/sanitize.dart';
 
 /// Ein Song aus der Subsonic-API
 class SubsonicSong {
@@ -63,6 +65,7 @@ class SubsonicAlbum {
 
 class NavidromeService {
   final DbHelper _db = DbHelper();
+  final FlutterSecureStorage _secure = const FlutterSecureStorage();
 
   String _serverUrl = '';
   String _user = '';
@@ -86,7 +89,7 @@ class NavidromeService {
       final prefs = await SharedPreferences.getInstance();
       final url = prefs.getString('navidrome_url');
       final user = prefs.getString('navidrome_user');
-      final pass = prefs.getString('navidrome_pass');
+      final pass = await _secure.read(key: 'navidrome_pass');
       if (url != null && user != null && pass != null && url.isNotEmpty) {
         setCredentials(url, user, pass);
       }
@@ -101,7 +104,7 @@ class NavidromeService {
       final prefs = await SharedPreferences.getInstance();
       await prefs.setString('navidrome_url', url);
       await prefs.setString('navidrome_user', user);
-      await prefs.setString('navidrome_pass', password);
+      await _secure.write(key: 'navidrome_pass', value: password);
     } catch (e) {
       debugPrint('Fehler beim Speichern der Navidrome-Zugangsdaten: $e');
     }
@@ -178,9 +181,9 @@ class NavidromeService {
       final musikDir = Directory('${dir.path}/music');
       if (!await musikDir.exists()) await musikDir.create(recursive: true);
 
-      final safeName = s.titel.replaceAll(RegExp(r'[^\w\s-]'), '').trim();
+      final safeName = sanitizeDateiname(s.titel);
       final kurzId = s.id.length > 8 ? s.id.substring(0, 8) : s.id;
-      final dateiName = '${safeName.isEmpty ? "song" : safeName}_$kurzId.mp3';
+      final dateiName = '${safeName}_$kurzId.mp3';
       final dateiPfad = '${musikDir.path}/$dateiName';
 
       final file = File(dateiPfad);