feat(05-03): implement seek slider with UAT-003 fix

- Add handleSeek with useCallback wrapper
- Implement UAT-003 fix pattern (pending seek while paused, immediate while playing)
- Store pending seek in pendingSeekRef when paused, update UI immediately
- Apply pending seek on resume in handlePlay (already implemented)
- Add seek slider with correct min/max/value bindings
- Disable slider during operations and before duration loaded
- Parse slider value to int before passing to handler
- Error handling for seek failures

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

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

@@ -183,6 +183,29 @@ const JKSessionJamTrackPlayer = ({
     }
   }, [isOperating, jamClient, dispatch]);
 
+  // Seek handler
+  const handleSeek = useCallback(async (newPositionMs) => {
+    if (isOperating || !jamClient || !fqIdRef.current) return;
+
+    try {
+      setError(null);
+
+      // UAT-003 fix: if paused, store pending seek
+      if (jamTrackState.isPaused) {
+        pendingSeekRef.current = newPositionMs;
+        dispatch(setJamTrackState({ currentPositionMs: newPositionMs }));
+        return;
+      }
+
+      // If playing, seek immediately
+      await jamClient.JamTrackSeekMs(fqIdRef.current, newPositionMs);
+      dispatch(setJamTrackState({ currentPositionMs: newPositionMs }));
+    } catch (err) {
+      console.error('[JamTrack] Seek error:', err);
+      setError({ type: 'playback', message: 'Failed to seek' });
+    }
+  }, [isOperating, jamClient, jamTrackState.isPaused, dispatch]);
+
   // Helper: Format milliseconds to MM:SS
   const formatTime = (ms) => {
     if (!ms || isNaN(ms)) return '00:00';
@@ -279,6 +302,18 @@ const JKSessionJamTrackPlayer = ({
         </button>
       </div>
 
+      <div>
+        <input
+          type="range"
+          min="0"
+          max={jamTrackState.durationMs || 100}
+          value={jamTrackState.currentPositionMs || 0}
+          onChange={(e) => handleSeek(parseInt(e.target.value, 10))}
+          disabled={isOperating || !jamTrackState.durationMs}
+          style={{ width: '300px' }}
+        />
+      </div>
+
       <div>
         <p>
           {formatTime(jamTrackState.currentPositionMs)} / {formatTime(jamTrackState.durationMs)}