feat(01-01): add 500ms playback monitoring with jamClient polling

- Polls jamClient.SessionCurrrentPlayPosMs() every 500ms when playing
- Updates currentTime and duration displays with formatted values
- Syncs isPlaying state with jamClient.isSessionTrackPlaying()
- Cleans up interval on pause/stop to prevent memory leaks
- Error handling logs to console but continues polling

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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

@@ -39,6 +39,41 @@ const JKSessionBackingTrackPlayer = ({
     }
   }, [isOpen, backingTrack]);
 
+  // Playback monitoring with 500ms polling
+  useEffect(() => {
+    let intervalId = null;
+
+    if (isPlaying && jamClient) {
+      // Start polling when playing
+      intervalId = setInterval(() => {
+        try {
+          if (!jamClient) return;
+
+          const positionMs = jamClient.SessionCurrrentPlayPosMs();
+          const durationMs = jamClient.SessionGetTracksPlayDurationMs();
+          const trackIsPlaying = jamClient.isSessionTrackPlaying();
+
+          setCurrentTime(formatTime(positionMs));
+          setDuration(formatTime(durationMs));
+
+          // Sync playing state if changed
+          if (trackIsPlaying !== isPlaying) {
+            setIsPlaying(trackIsPlaying);
+          }
+        } catch (error) {
+          console.error('Error polling playback status:', error);
+        }
+      }, 500);
+    }
+
+    // Cleanup: clear interval on pause/stop or unmount
+    return () => {
+      if (intervalId) {
+        clearInterval(intervalId);
+      }
+    };
+  }, [isPlaying, jamClient, backingTrack]);
+
   const handlePlay = async () => {
     try {
       if (isPlaying) {