fix(26-03): remove 'idle' from stems render condition in JKSessionScreen

- Change condition from 'synchronized' OR 'idle' to only 'synchronized'
- Stems only appear after JamTrack is synchronized with backend
- Already-synchronized tracks show stems immediately (checkJamTrackSync returns synchronized)

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

@@ -1415,9 +1415,9 @@ const JKSessionScreen = () => {
             </div>
           </div>
 
-          {/* JamTrack Section - Show stems when player is ready */}
+          {/* JamTrack Section - Show stems only when synchronized with backend */}
           {selectedJamTrack && jamTrackStems.length > 0 &&
-            (jamTrackDownloadState.state === 'synchronized' || jamTrackDownloadState.state === 'idle') && (
+            (jamTrackDownloadState.state === 'synchronized') && (
               <>
                 <div style={{ borderLeft: '1px solid #ddd', paddingLeft: '1rem' }}></div>
                 <div className='jamTrack'>