feat(queue): add source_item_id_hint to QueueTrack

Reserved for upcoming Mixtape/Collection enqueue paths to mark item
boundaries. Existing enqueue paths populate it with the track's
album_id as a safe fallback so skip-to-item commands behave
correctly even for queues built via "play album" today.

cc

crates/qbz-player/src/queue.rs

@@ -900,6 +900,7 @@ mod tests {
             streamable: true,
             source: Some("test".to_string()),
             parental_warning: false,
+            source_item_id_hint: None,
         }
     }