feat(01-01): initialize duration immediately when track loads

- Fetches duration via jamClient.SessionGetTracksPlayDurationMs() on open
- Displays duration before playback starts (e.g., "5:13" shown immediately)
- Error handling falls back to "0:00" if fetch fails
- Matches legacy web behavior of showing duration on load

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

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

@@ -31,13 +31,21 @@ const JKSessionBackingTrackPlayer = ({
   };
 
   useEffect(() => {
-    if (isOpen && backingTrack) {
+    if (isOpen && backingTrack && jamClient) {
       // Initialize player state when opened
       setIsPlaying(false);
       setCurrentTime('0:00');
-      setDuration('0:00');
+
+      // Fetch and set duration immediately when track loads
+      try {
+        const durationMs = jamClient.SessionGetTracksPlayDurationMs();
+        setDuration(formatTime(durationMs));
+      } catch (error) {
+        console.error('Error fetching track duration:', error);
+        setDuration('0:00');
+      }
     }
-  }, [isOpen, backingTrack]);
+  }, [isOpen, backingTrack, jamClient]);
 
   // Playback monitoring with 500ms polling
   useEffect(() => {