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/audio_handler.dart

@@ -54,20 +54,20 @@ class MeloAudioHandler extends BaseAudioHandler {
   Future<void> play() async {
     if (_player.aktuellerSong != null) {
       _playing = true;
-      await _player.playPause();
+      await _player.play();
     }
   }
 
   @override
   Future<void> pause() async {
     _playing = false;
-    await _player.playPause();
+    await _player.pause();
   }
 
   @override
   Future<void> stop() async {
     _playing = false;
-    await _player.playPause();
+    await _player.pause();
     playbackState.add(playbackState.value.copyWith(
       controls: [MediaControl.play],
       playing: false,
@@ -77,7 +77,7 @@ class MeloAudioHandler extends BaseAudioHandler {
 
   @override
   Future<void> seek(Duration position) async {
-    _player.spiele(_player.aktuellerSong!, position: position.inSeconds);
+    await _player.seek(position);
   }
 
   @override