fix(03-03): resolve end-of-track and popup mode issues

Issue 1 - End of track playback:
- When track ends, call SessionStopPlay() and reset position to 0
- Ensures clean state for next playback
- Fixes "3 click" bug when restarting after track ends

Issue 2 & 3 - Volume and loop not working in popup mode:
- Add getBackingTrackPath() helper to handle both formats
- In popup mode, backingTrack is a string (path)
- In modal mode, backingTrack is an object with .path property
- Update handleLoopChange to use helper
- Fixes "No backing track available" error

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

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

@@ -172,7 +172,14 @@ const JKSessionBackingTrackPlayer = ({
 
           // Check if track reached the end
           if (validPosition >= validDuration && validDuration > 0) {
+            // Stop playback and reset to beginning for clean state
+            await jamClient.SessionStopPlay();
             setIsPlaying(false);
+            setCurrentPositionMs(0);
+            setCurrentTime('0:00');
+            // Exit early - don't update position to end
+            consecutivePollingErrorsRef.current = 0;
+            return;
           }
 
           // Only update state if values actually changed (lazy updates)
@@ -374,18 +381,19 @@ const JKSessionBackingTrackPlayer = ({
 
       // Set loop state using SessionSetBackingTrackFileLoop
       // This requires the file path and loop boolean
-      if (!backingTrack || !backingTrack.path) {
+      const trackPath = getBackingTrackPath(backingTrack);
+      if (!trackPath) {
         handleError('playback', 'No backing track available for loop change', null);
         setIsLooping(previousLoop); // Revert checkbox
         return;
       }
 
-      await jamClient.SessionSetBackingTrackFileLoop(backingTrack.path, shouldLoop);
+      await jamClient.SessionSetBackingTrackFileLoop(trackPath, shouldLoop);
     } catch (error) {
       handleError('playback', 'Failed to toggle loop', error);
       setIsLooping(previousLoop); // Revert checkbox on error
     }
-  }, [isLooping, jamClient, backingTrack, handleError]);
+  }, [isLooping, jamClient, backingTrack, handleError, getBackingTrackPath]);
 
   /*
    * UAT-003 Fix: Seek while paused limitation workaround
@@ -460,18 +468,30 @@ const JKSessionBackingTrackPlayer = ({
     }
   }, [handleStop, closeMedia, isPopup, onClose]);
 
+  // Helper to get backing track path (handles both string and object formats)
+  const getBackingTrackPath = useCallback((backingTrack) => {
+    if (!backingTrack) return null;
+    // If backingTrack is a string, it's the path itself (popup mode)
+    if (typeof backingTrack === 'string') return backingTrack;
+    // If it's an object, extract the path property (modal mode)
+    return backingTrack.path || null;
+  }, []);
+
   const getFileName = useCallback((backingTrack) => {
     if (!backingTrack) return 'Unknown File';
 
-    // Extract filename from path or use provided name
-    if (backingTrack.path) {
-      return backingTrack.path.split('/').pop().split('\\').pop();
+    // Get the path first
+    const path = getBackingTrackPath(backingTrack);
+    if (path) {
+      return path.split('/').pop().split('\\').pop();
     }
-    if (backingTrack.name) {
+
+    // Fallback to name if it's an object
+    if (typeof backingTrack === 'object' && backingTrack.name) {
       return backingTrack.name;
     }
     return 'Audio File';
-  }, []);
+  }, [getBackingTrackPath]);