feat(queue): expose stop-after marker through QueueState

The marker becomes part of the queue state snapshot the frontend
already consumes via syncQueueState(). Single source of truth — no
parallel event channel, no separate store on the frontend.

Refs #356.

cc

crates/qbz-player/src/queue.rs

@@ -858,6 +858,7 @@ impl QueueManager {
             shuffle: state.shuffle,
             repeat: state.repeat,
             total_tracks: state.tracks.len(),
+            stop_after_track_id: state.stop_after_track_id,
         }
     }
 
@@ -1815,4 +1816,25 @@ mod tests {
 
         assert_eq!(queue.get_stop_after(), Some(102));
     }
+
+    #[test]
+    fn test_get_state_includes_stop_after() {
+        let queue = QueueManager::new();
+        queue.add_track(create_test_track(101));
+        queue.set_stop_after(101);
+
+        let state = queue.get_state();
+
+        assert_eq!(state.stop_after_track_id, Some(101));
+    }
+
+    #[test]
+    fn test_get_state_returns_none_when_no_marker() {
+        let queue = QueueManager::new();
+        queue.add_track(create_test_track(101));
+
+        let state = queue.get_state();
+
+        assert_eq!(state.stop_after_track_id, None);
+    }
 }