fix(player): queue skip + cut over silence on track tap

Two bugs in the audio handler caused the playback issues seen on the
v2026.05.13.0 build:

1. **Queue button on the now-playing screen did nothing.** MinstrelAudio
   Handler never overrode skipToQueueItem, so taps in QueueScreen fell
   through to BaseAudioHandler's empty default. The queue UI updated
   nothing because the handler did nothing. Now overridden: rebuilds
   the source list via setQueueFromTracks(_lastTracks, initialIndex)
   so the targeted item plays cleanly even when its source hadn't been
   built yet by the background fill.

2. **Tapping a song in a playlist let the previous track bleed through
   until the new source finished building.** setQueueFromTracks awaits
   _buildAudioSource before swapping the player's source list, and
   that wait can be a few hundred ms on a cache miss. During the wait
   the old source kept playing while the mini player UI had already
   flipped to the new title/artist. Now pause()ing the player at the
   start of setQueueFromTracks silences the old source the moment the
   user taps.

Also stashes the most recent TrackRef list as _lastTracks so
skipToQueueItem can reconstruct sources without having to peek into
just_audio's internal source list.

flutter_client/lib/player/audio_handler.dart

@@ -68,6 +68,14 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
   /// new queue, leaving the player "locked" to a corrupted state.
   int _queueGeneration = 0;
 
+  /// Tracks the most recent setQueueFromTracks() input so
+  /// skipToQueueItem can reconstruct the source list. just_audio
+  /// requires every source to be built before it can be a skip
+  /// target, but setQueueFromTracks only builds the initial source
+  /// and fills the rest in the background — so a skip to an index
+  /// past the fill front needs to rebuild from the stored tracks.
+  List<TrackRef> _lastTracks = const [];
+
   /// Volume stream for UI subscribers. Mirrors the just_audio player's
   /// volume directly; set via setVolume(double).
   Stream<double> get volumeStream => _player.volumeStream;
@@ -105,6 +113,16 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
     // Reset suppress flag in case a prior backward-fill bailed on
     // gen check before reaching its `finally`.
     _suppressIndexUpdates = false;
+    _lastTracks = tracks;
+
+    // Pause the old source immediately so the previous track stops
+    // audibly the moment the user taps, instead of bleeding through
+    // until the new source finishes building. setAudioSources below
+    // will swap the source list cleanly; pause is the simplest way
+    // to silence the player during the (possibly multi-100ms) build.
+    if (_player.playing) {
+      await _player.pause();
+    }
 
     // Populate the visible queue + current mediaItem immediately so
     // the player UI reflects the user's tap before any source has
@@ -130,6 +148,23 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
     unawaited(_fillRemainingSources(tracks, clampedInitial, myGen));
   }
 
+  /// Switches playback to the [index]th queue item. The full
+  /// just_audio source list isn't necessarily built yet
+  /// (_fillRemainingSources runs in the background), so we
+  /// reconstruct from the stored TrackRef list rather than calling
+  /// _player.seek(index: ...) on a possibly-missing source. Calling
+  /// setQueueFromTracks again is the safe path: it bumps the
+  /// generation, cancels any in-flight fill, rebuilds source[0] as
+  /// the target, and re-fills around. play() restarts playback so
+  /// queue taps feel like "jump to this song" rather than "set the
+  /// pointer and wait for me to press play."
+  @override
+  Future<void> skipToQueueItem(int index) async {
+    if (index < 0 || index >= _lastTracks.length) return;
+    await setQueueFromTracks(_lastTracks, initialIndex: index);
+    await play();
+  }
+
   /// Background fill of the rest of the just_audio source list after
   /// the initial source is playing. Forward direction first (most
   /// common skipNext target). Backward inserts shift the player's