feat(locallibrary): album/list play + source-aware auto-advance (playback Stage 2)

A Local Library album, or the Tracks list, now plays through with auto-advance
across mixed local/offline sources.

The one change to the shared playback path is additive and provably safe for
Qobuz: play_audible gains a guard at the top — when the CURRENT queue track's
source is "local" (and its id matches), it plays the file from disk via
play_local_file_audible (read -> player.play_data + CUE seek); otherwise it
falls through to the EXISTING offline/Qobuz tier-walk, byte-for-byte unchanged.
Qobuz/offline queue tracks never carry source "local", so their behavior cannot
change. Auto-advance, next/previous and play-all all route through play_audible,
so they become source-aware for free. current_track() releases the queue read
lock before play (no nested lock / deadlock).

- qbz-player: QueueManager::current() (additive).
- qbz-core: QbzCore::current_track() (additive).
- qbz-slint playback.rs: source-aware play_audible + play_local_file_audible
  (extracted) + play_local_tracks_now + play_local_album
  (get_album_tracks_metadata) + play_local_tracks_from (search_with_filter);
  replaces the Stage-1 single-track helper.
- main.rs: album card "play" -> play the album; Tracks row "play" -> play from
  that row down the searched list. (play-next / queue still stubbed.)

Built green (cargo check). Local-file gapless isn't wired (local tracks aren't
prefetched, so local/mixed transitions use the simple track-ended -> next path);
Qobuz gapless is unchanged. Runtime smoke-test pending: (1) Qobuz streaming
still plays + advances, (2) an offline album advances, (3) a local album/list
advances.

cc

crates/qbz-core/src/core.rs

@@ -600,6 +600,12 @@ impl<A: FrontendAdapter + Send + Sync + 'static> QbzCore<A> {
         queue.peek_upcoming(count)
     }
 
+    /// The current queue track, if any (source-aware playback routing).
+    pub async fn current_track(&self) -> Option<QueueTrack> {
+        let queue = self.queue.read().await;
+        queue.current()
+    }
+
     // ==================== Search & Catalog ====================
 
     /// Search for albums