feat(05-03): implement playback control handlers (play/pause/stop)

- Add handlePlay with loadJamTrack integration and pause-resume logic
- Add handlePause for pausing playback
- Add handleStop for stopping and resetting position
- Implement UAT-003 fix pattern (pendingSeekRef for pause-seek-resume)
- Add isOperating flag to prevent rapid clicks
- Render playback buttons with proper disabled states
- Error handling with typed errors (file/network red, playback yellow)

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

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

@@ -34,6 +34,7 @@ const JKSessionJamTrackPlayer = ({
   // Refs
   const fqIdRef = useRef(null);
   const mountedRef = useRef(true);
+  const pendingSeekRef = useRef(null);
 
   // Helper: Build fqId
   const buildFqId = useCallback(async () => {
@@ -102,18 +103,118 @@ const JKSessionJamTrackPlayer = ({
     };
   }, [dispatch, jamClient]);
 
-  // Placeholder render (will be filled in Plan 3)
+  // Playback control handlers
+  const handlePlay = useCallback(async () => {
+    if (isOperating || !jamClient || !fqIdRef.current) return;
+
+    try {
+      setIsOperating(true);
+      setError(null);
+
+      // If not playing yet, load JamTrack
+      if (!jamTrackState.isPlaying) {
+        await dispatch(loadJamTrack({
+          jamTrack,
+          mixdownId: selectedMixdownId,
+          autoPlay: true,
+          jamClient
+        })).unwrap();
+      } else if (jamTrackState.isPaused) {
+        // Resume from pause
+        await jamClient.JamTrackResume(fqIdRef.current);
+
+        // Apply pending seek if exists (UAT-003 fix)
+        if (pendingSeekRef.current !== null) {
+          await jamClient.JamTrackSeekMs(fqIdRef.current, pendingSeekRef.current);
+          pendingSeekRef.current = null;
+        }
+
+        dispatch(setJamTrackState({ isPaused: false, isPlaying: true }));
+      }
+    } catch (err) {
+      console.error('[JamTrack] Play error:', err);
+      setError({ type: 'playback', message: 'Failed to play JamTrack' });
+    } finally {
+      if (mountedRef.current) {
+        setIsOperating(false);
+      }
+    }
+  }, [isOperating, jamClient, jamTrack, jamTrackState, selectedMixdownId, dispatch]);
+
+  const handlePause = useCallback(async () => {
+    if (isOperating || !jamClient || !fqIdRef.current) return;
+
+    try {
+      setIsOperating(true);
+      setError(null);
+
+      await jamClient.JamTrackPause(fqIdRef.current);
+      dispatch(setJamTrackState({ isPaused: true, isPlaying: false }));
+    } catch (err) {
+      console.error('[JamTrack] Pause error:', err);
+      setError({ type: 'playback', message: 'Failed to pause JamTrack' });
+    } finally {
+      if (mountedRef.current) {
+        setIsOperating(false);
+      }
+    }
+  }, [isOperating, jamClient, dispatch]);
+
+  const handleStop = useCallback(async () => {
+    if (isOperating || !jamClient || !fqIdRef.current) return;
+
+    try {
+      setIsOperating(true);
+      setError(null);
+
+      await jamClient.JamTrackStop(fqIdRef.current);
+      dispatch(setJamTrackState({
+        isPlaying: false,
+        isPaused: false,
+        currentPositionMs: 0
+      }));
+    } catch (err) {
+      console.error('[JamTrack] Stop error:', err);
+      setError({ type: 'playback', message: 'Failed to stop JamTrack' });
+    } finally {
+      if (mountedRef.current) {
+        setIsOperating(false);
+      }
+    }
+  }, [isOperating, jamClient, dispatch]);
+
   if (!isOpen && !isPopup) return null;
 
   return (
     <div className="jamtrack-player">
-      <h3>JamTrack Player (Skeleton)</h3>
-      {isLoadingSync && <p>Checking sync status...</p>}
-      {error && <p style={{ color: 'red' }}>{error.message}</p>}
-      {downloadState.state !== 'idle' && (
-        <p>Download State: {downloadState.state} ({downloadState.progress}%)</p>
+      {error && (
+        <div style={{ background: error.type === 'file' || error.type === 'network' ? '#fee' : '#ffd', padding: '10px' }}>
+          {error.message}
+          <button onClick={() => setError(null)}>Dismiss</button>
+        </div>
       )}
-      <p>Ready for playback controls (Plan 3)</p>
+
+      {downloadState.state !== 'idle' && downloadState.state !== 'synchronized' && (
+        <div>
+          <p>Download: {downloadState.state} ({downloadState.progress}%)</p>
+        </div>
+      )}
+
+      <div>
+        <button onClick={handlePlay} disabled={isOperating || isLoadingSync}>
+          {jamTrackState.isPaused ? 'Resume' : 'Play'}
+        </button>
+        <button onClick={handlePause} disabled={isOperating || !jamTrackState.isPlaying}>
+          Pause
+        </button>
+        <button onClick={handleStop} disabled={isOperating || (!jamTrackState.isPlaying && !jamTrackState.isPaused)}>
+          Stop
+        </button>
+      </div>
+
+      <div>
+        <p>Position: {jamTrackState.currentPositionMs}ms / Duration: {jamTrackState.durationMs}ms</p>
+      </div>
     </div>
   );
 };