Prevent auto-advance on user-requested stop

Mark user-requested stops to prevent auto-advance when sleep timer expires. This ensures that playback stops cleanly without triggering automatic advancement to the next item.

pmocontrol/src/control_point.rs

@@ -542,6 +542,9 @@ impl ControlPoint {
                             "Sleep timer expired, stopping playback"
                         );
 
+                        // Mark this as a user-requested stop to prevent auto-advance
+                        renderer.mark_user_stop_requested();
+
                         // Stop playback
                         if let Err(err) = renderer.stop() {
                             warn!(