fix(27-01): add ignore flag to prevent unmount state updates

- Add let ignore = false at start of duration fetch useEffect
- Check ignore flag after async jamClient call
- Add cleanup function that sets ignore = true
- Prevents 'state update on unmounted component' React warnings
  when closing backing track popup quickly

jam-ui/src/components/client/JKSessionBackingTrackPlayer.js

@@ -88,6 +88,8 @@ const JKSessionBackingTrackPlayer = ({
   }, []);
 
   useEffect(() => {
+    let ignore = false;
+
     // For popup mode, treat as always "open" since the popup window itself indicates open state
     const shouldInitialize = (isOpen || isPopup) && backingTrack && jamClient;
 
@@ -99,16 +101,23 @@ const JKSessionBackingTrackPlayer = ({
 
       // Fetch and set duration immediately when track loads (async)
       const fetchDuration = async () => {
+        if (ignore) return;
         setIsLoadingDuration(true);
+
         try {
           if (!jamClient) {
+            if (!ignore) {
               handleError('general', 'Audio engine not available', null);
               setIsLoadingDuration(false);
+            }
             return;
           }
 
           const durationInMs = await jamClient.SessionGetTracksPlayDurationMs();
 
+          // Check ignore AFTER async operation
+          if (ignore) return;
+
           // Convert string to number (jamClient returns string values)
           const validDuration = parseInt(durationInMs, 10) || 0;
 
@@ -131,15 +140,21 @@ const JKSessionBackingTrackPlayer = ({
           clearError(); // Clear any previous errors
           setIsLoadingDuration(false);
         } catch (error) {
+          if (!ignore) {
             handleError('file', 'Failed to load track duration', error);
             setDuration('0:00');
             setDurationMs(0);
             setIsLoadingDuration(false);
           }
+        }
       };
 
       fetchDuration();
     }
+
+    return () => {
+      ignore = true;
+    };
   }, [isOpen, isPopup, backingTrack, jamClient, handleError, formatTime, clearError]);
 
   // Playback monitoring with visibility-aware polling