feat(05-04): implement mixdown picker UI with hierarchy sorting

- Add handleMixdownChange callback to switch between mixdowns
- Stop and restart playback when changing mixdown (if playing)
- Add mixdown dropdown selector with sorted display
- Sort order: master → custom mixes → stems (alphabetical within types)
- Visual indicators: 🎵 master, 🎨 custom, 🎸 stem
- Disable picker during operations and loading
- Update selectedMixdownId local state and activeMixdown Redux state

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

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

@@ -241,6 +241,46 @@ const JKSessionJamTrackPlayer = ({
     }
   }, [isOperating, jamClient, jamTrackState.isPaused, dispatch]);
 
+  // Mixdown change handler
+  const handleMixdownChange = useCallback(async (mixdownId) => {
+    if (isOperating || !jamClient || !fqIdRef.current) return;
+
+    try {
+      setIsOperating(true);
+      setError(null);
+
+      // Find the new mixdown
+      const mixdown = availableMixdowns.find(m => m.id === mixdownId);
+      if (!mixdown) {
+        throw new Error('Mixdown not found');
+      }
+
+      // Update local state
+      setSelectedMixdownId(mixdownId);
+      dispatch(setActiveMixdown(mixdown));
+
+      // If currently playing, stop and restart with new mixdown
+      if (jamTrackState.isPlaying || jamTrackState.isPaused) {
+        await jamClient.JamTrackStop(fqIdRef.current);
+
+        await dispatch(loadJamTrack({
+          jamTrack,
+          mixdownId,
+          autoPlay: true,
+          jamClient
+        })).unwrap();
+      }
+
+    } catch (err) {
+      console.error('[JamTrack] Mixdown change error:', err);
+      setError({ type: 'playback', message: 'Failed to change mixdown' });
+    } finally {
+      if (mountedRef.current) {
+        setIsOperating(false);
+      }
+    }
+  }, [isOperating, jamClient, jamTrack, jamTrackState, availableMixdowns, dispatch]);
+
   // Helper: Format milliseconds to MM:SS
   const formatTime = (ms) => {
     if (!ms || isNaN(ms)) return '00:00';
@@ -354,6 +394,37 @@ const JKSessionJamTrackPlayer = ({
           {formatTime(jamTrackState.currentPositionMs)} / {formatTime(jamTrackState.durationMs)}
         </p>
       </div>
+
+      {availableMixdowns.length > 0 && (
+        <div style={{ marginTop: '10px' }}>
+          <label>Mixdown:</label>
+          <select
+            value={selectedMixdownId || ''}
+            onChange={(e) => handleMixdownChange(parseInt(e.target.value, 10))}
+            disabled={isOperating || isLoadingSync}
+          >
+            {availableMixdowns
+              .slice()
+              .sort((a, b) => {
+                // Sort order: master first, then custom mixes, then stems
+                if (a.type === 'master') return -1;
+                if (b.type === 'master') return 1;
+                if (a.type === 'custom' || a.type === 'custom-mix') return -1;
+                if (b.type === 'custom' || b.type === 'custom-mix') return 1;
+                return a.name.localeCompare(b.name);
+              })
+              .map(mixdown => (
+                <option key={mixdown.id} value={mixdown.id}>
+                  {mixdown.type === 'master' && '🎵 '}
+                  {(mixdown.type === 'custom' || mixdown.type === 'custom-mix') && '🎨 '}
+                  {mixdown.type === 'stem' && '🎸 '}
+                  {mixdown.name}
+                </option>
+              ))
+            }
+          </select>
+        </div>
+      )}
     </div>
   );
 };