fix(uat-02-01): parse string values and add playback mode parameter

Fixes UAT-002 (continued): Slider not moving, time not updating

Two issues discovered:
1. jamClient returns string values ('0', '228818') not numbers
2. SessionStartPlay() requires playback mode parameter (missing in our code)

Solution:
- Use parseInt() to convert jamClient string returns to numbers
- Pass playback mode parameter 1 (normal playback) to SessionStartPlay()
- Matches legacy implementation: SessionStartPlay(1) or SessionStartPlay(data.playbackMode)

This should enable:
- Slider to move during playback (now has numeric position values)
- Audio to actually start playing (correct API call with parameter)
- Time displays to update correctly

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

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

@@ -49,8 +49,8 @@ const JKSessionBackingTrackPlayer = ({
           const durationInMs = await jamClient.SessionGetTracksPlayDurationMs();
           console.log('JKSessionBackingTrackPlayer: Duration from jamClient:', durationInMs, 'Type:', typeof durationInMs);
 
-          // Ensure we have a valid number
-          const validDuration = (durationInMs != null && !isNaN(durationInMs) && durationInMs >= 0) ? durationInMs : 0;
+          // Convert string to number (jamClient returns string values)
+          const validDuration = parseInt(durationInMs, 10) || 0;
           setDurationMs(validDuration);
           setDuration(formatTime(validDuration));
         } catch (error) {
@@ -81,9 +81,9 @@ const JKSessionBackingTrackPlayer = ({
 
           console.log('JKSessionBackingTrackPlayer polling:', { positionMs, durationInMs, trackIsPlaying });
 
-          // Ensure valid numbers
-          const validPosition = (positionMs != null && !isNaN(positionMs) && positionMs >= 0) ? positionMs : 0;
-          const validDuration = (durationInMs != null && !isNaN(durationInMs) && durationInMs >= 0) ? durationInMs : 0;
+          // Convert strings to numbers (jamClient returns string values)
+          const validPosition = parseInt(positionMs, 10) || 0;
+          const validDuration = parseInt(durationInMs, 10) || 0;
 
           setCurrentPositionMs(validPosition);
           setCurrentTime(formatTime(validPosition));
@@ -115,8 +115,8 @@ const JKSessionBackingTrackPlayer = ({
         await jamClient.SessionPausePlay();
         setIsPlaying(false);
       } else {
-        // Play
-        await jamClient.SessionStartPlay();
+        // Play (1 = normal playback mode)
+        await jamClient.SessionStartPlay(1);
         setIsPlaying(true);
       }
     } catch (error) {