feat(05-03): show JamTrack stems when player is ready

Shows JamTrack stems in session screen only when the player is ready
to play (after download/packaging is complete), matching backing track
behavior.

Changes:
- Added jamTrackDownloadState selector
- Updated conditional render to check download state
- Stems now show when state is 'synchronized' or 'idle'
- Prevents showing stems during packaging/downloading

Stems display flow:
1. User selects JamTrack from modal
2. handleJamTrackSelect fetches JamTrack with stems
3. Sets selectedJamTrack and jamTrackStems in Redux
4. Player opens and begins packaging/downloading if needed
5. Once downloadState becomes 'synchronized', stems appear in session
6. Clicking close (player or session) removes stems and closes player

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

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

@@ -209,6 +209,7 @@ const JKSessionScreen = () => {
   // Redux jam track state
   const selectedJamTrack = useSelector(selectSelectedJamTrack);
   const jamTrackStems = useSelector(selectJamTrackStems);
+  const jamTrackDownloadState = useSelector(state => state.media.downloadState);
 
   // JamTrack close handler (used by both player and session screen)
   const handleJamTrackClose = useCallback(async () => {
@@ -1067,8 +1068,9 @@ const JKSessionScreen = () => {
             </div>
           </div>
 
-          {/* JamTrack Section */}
-          {selectedJamTrack && jamTrackStems.length > 0 && (
+          {/* JamTrack Section - Show stems when player is ready */}
+          {selectedJamTrack && jamTrackStems.length > 0 &&
+           (jamTrackDownloadState.state === 'synchronized' || jamTrackDownloadState.state === 'idle') && (
             <>
               <div style={{ borderLeft: '1px solid #ddd', paddingLeft: '1rem' }}></div>
               <div className='jamTrack'>