fix(02-01): handle seek to end edge case in backing track player

UAT-004: When seeking to end while paused, then clicking play, the
player would break (slider jumps to 0, no audio, play button stops working).

Root cause: jamClient enters end-of-track state when position >= duration.
Calling SessionStartPlay() without resetting causes undefined behavior.

Solution:
- Detect when position is at/near end (within 100ms)
- Call SessionStopPlay() to reset jamClient state
- Seek to position 0
- Then start playback normally

Also added diagnostic logging to handlePlay, handleSeek, and polling
to help debug playback state transitions.

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

@@ -85,6 +85,12 @@ const JKSessionBackingTrackPlayer = ({
           const validPosition = parseInt(positionMs, 10) || 0;
           const validDuration = parseInt(durationInMs, 10) || 0;
 
+          // Check if track reached the end
+          if (validPosition >= validDuration && validDuration > 0) {
+            console.log('JKSessionBackingTrackPlayer: Track reached end, stopping playback');
+            setIsPlaying(false);
+          }
+
           setCurrentPositionMs(validPosition);
           setCurrentTime(formatTime(validPosition));
           setDurationMs(validDuration);
@@ -92,6 +98,7 @@ const JKSessionBackingTrackPlayer = ({
 
           // Sync playing state if changed
           if (trackIsPlaying !== isPlaying) {
+            console.log('JKSessionBackingTrackPlayer: Playing state changed from', isPlaying, 'to', trackIsPlaying);
             setIsPlaying(trackIsPlaying);
           }
         } catch (error) {
@@ -110,14 +117,34 @@ const JKSessionBackingTrackPlayer = ({
 
   const handlePlay = async () => {
     try {
+      console.log('JKSessionBackingTrackPlayer handlePlay:', {
+        isPlaying,
+        currentPositionMs,
+        durationMs
+      });
+
       if (isPlaying) {
         // Pause
         await jamClient.SessionPausePlay();
         setIsPlaying(false);
+        console.log('JKSessionBackingTrackPlayer: Paused');
       } else {
+        // Check if we're at or very near the end (within 100ms)
+        if (currentPositionMs >= durationMs - 100 && durationMs > 0) {
+          console.log('JKSessionBackingTrackPlayer: At end of track, resetting to start before play');
+          // Stop first to reset state
+          await jamClient.SessionStopPlay();
+          // Seek to start
+          await jamClient.SessionTrackSeekMs(0);
+          // Update UI state
+          setCurrentPositionMs(0);
+          setCurrentTime('0:00');
+        }
+
         // Play (1 = normal playback mode)
         await jamClient.SessionStartPlay(1);
         setIsPlaying(true);
+        console.log('JKSessionBackingTrackPlayer: Playing');
       }
     } catch (error) {
       console.error('Error toggling playback:', error);
@@ -175,12 +202,21 @@ const JKSessionBackingTrackPlayer = ({
     const seekPositionMs = parseInt(e.target.value);
 
     try {
+      console.log('JKSessionBackingTrackPlayer handleSeek:', {
+        seekPositionMs,
+        durationMs,
+        isPlaying,
+        currentPositionMs
+      });
+
       // Update local state immediately for responsive UI
       setCurrentPositionMs(seekPositionMs);
       setCurrentTime(formatTime(seekPositionMs));
 
       // Seek the native client to the new position
       await jamClient.SessionTrackSeekMs(seekPositionMs);
+
+      console.log('JKSessionBackingTrackPlayer: Seek completed');
     } catch (error) {
       console.error('Error seeking:', error);
     }