feat(navidrome): Erweiterte Service-APIs (Artists, Favoriten)

- getArtists(): Alle Künstler vom Server laden
- getArtistSongs(): Alle Songs eines Künstlers
- setFavorite()/removeFavorite(): Server-Favoriten toggle
- getFavorites(): Favoriten vom Server laden
- SubsonicArtist-Klasse für Künstler

APIs für Phase 2 vorbereitet (Artist-Tab, Server-Favoriten-Sync)
Tests: 70/70 grün

lib/services/navidrome_service.dart

@@ -178,4 +178,92 @@ class NavidromeService {
   Uri? coverUrl(String coverId) {
     return _uri('getCoverArt.view', {'id': coverId});
   }
+
+  Future<bool> setFavorite(String songId) async {
+    try {
+      final r = await http.get(_uri('star.view', {'id': songId})).timeout(const Duration(seconds: 10));
+      return r.statusCode == 200;
+    } catch (e) {
+      debugPrint('Navidrome setFavorite Fehler: $e');
+      return false;
+    }
+  }
+
+  Future<bool> removeFavorite(String songId) async {
+    try {
+      final r = await http.get(_uri('unstar.view', {'id': songId})).timeout(const Duration(seconds: 10));
+      return r.statusCode == 200;
+    } catch (e) {
+      debugPrint('Navidrome removeFavorite Fehler: $e');
+      return false;
+    }
+  }
+
+  Future<List<SubsonicSong>> getFavorites() async {
+    try {
+      final r = await http.get(_uri('getStarred.view')).timeout(const Duration(seconds: 15));
+      if (r.statusCode != 200) return [];
+      final data = jsonDecode(r.body);
+      final songs = data['subsonic-response']?['starred']?['song'] as List? ?? [];
+      return songs.map((j) => SubsonicSong.fromJson(j)).toList();
+    } catch (e) {
+      debugPrint('Navidrome getFavorites Fehler: $e');
+      return [];
+    }
+  }
+
+  Future<List<SubsonicArtist>> getArtists() async {
+    try {
+      final r = await http.get(_uri('getArtists.view')).timeout(const Duration(seconds: 15));
+      if (r.statusCode != 200) return [];
+      final data = jsonDecode(r.body);
+      final indexData = data['subsonic-response']?['artists']?['index'] as List? ?? [];
+      final artists = <SubsonicArtist>[];
+      for (final idx in indexData) {
+        final artistList = idx['artist'] as List? ?? [];
+        for (final a in artistList) {
+          artists.add(SubsonicArtist.fromJson(a));
+        }
+      }
+      return artists;
+    } catch (e) {
+      debugPrint('Navidrome getArtists Fehler: $e');
+      return [];
+    }
+  }
+
+  Future<List<SubsonicSong>> getArtistSongs(String artistId) async {
+    try {
+      final r = await http.get(_uri('getArtist.view', {'id': artistId})).timeout(const Duration(seconds: 15));
+      if (r.statusCode != 200) return [];
+      final data = jsonDecode(r.body);
+      final albums = data['subsonic-response']?['artist']?['album'] as List? ?? [];
+      final songs = <SubsonicSong>[];
+      for (final album in albums) {
+        final albumSongs = album['song'] as List? ?? [];
+        for (final song in albumSongs) {
+          songs.add(SubsonicSong.fromJson(song));
+        }
+      }
+      return songs;
+    } catch (e) {
+      debugPrint('Navidrome getArtistSongs Fehler: $e');
+      return [];
+    }
+  }
+}
+
+class SubsonicArtist {
+  final String id;
+  final String name;
+
+  const SubsonicArtist({
+    required this.id,
+    required this.name,
+  });
+
+  factory SubsonicArtist.fromJson(Map<String, dynamic> j) => SubsonicArtist(
+    id: j['id'] as String,
+    name: j['name'] as String? ?? 'Unbekannt',
+  );
 }