feat(flutter/player): playNext + enqueue + PlaylistsApi.appendTracks

Three small data-layer additions for the upcoming track-actions menu:

- PlaylistsApi.appendTracks(playlistId, trackIds) wraps
  POST /api/playlists/{id}/tracks for the "Add to playlist" action.
- audio_handler gains playNext (insertAudioSource at currentIndex+1)
  and enqueue (addAudioSource) — both also push the audio_service
  queue notifier so the queue-screen UI stays in sync.
- The AudioSource construction was extracted into a private
  _buildAudioSource helper so setQueueFromTracks / playNext / enqueue
  share one source-building path.
- PlayerActions exposes playNext / enqueue for menu use.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/api/endpoints/playlists.dart

@@ -44,4 +44,13 @@ class PlaylistsApi {
     final r = await _dio.get<Map<String, dynamic>>('/api/playlists/$id');
     return PlaylistDetail.fromJson(r.data ?? const {});
   }
+
+  /// POST /api/playlists/{id}/tracks. Owner only; server returns the
+  /// playlist detail with the new rows.
+  Future<void> appendTracks(String playlistId, List<String> trackIds) async {
+    await _dio.post<void>(
+      '/api/playlists/$playlistId/tracks',
+      data: {'track_ids': trackIds},
+    );
+  }
 }