fix: pending drag effect calls stale handleSectionItemDragStart

The useEffect with [] deps captured handleSectionItemDragStart
from initial render, which read an empty selectedItemIds.
Added handleSectionItemDragStartRef that stays current across
renders; the effect calls ref.current instead of the closure.

app/static/js/app.jsx

@@ -6948,6 +6948,7 @@ const App = () => {
   const sweepTrackIdRef = useRef(null);
   const sweepSelectRef = useRef(null);
   const pendingDragRef = useRef(null); // { trackId, itemType, itemId, clickOffset, startX, startY }
+  const handleSectionItemDragStartRef = useRef(null);
   const [localSelectionTrackId, setLocalSelectionTrackId] = useState(null);
   const [localSelectionStart, setLocalSelectionStart] = useState(null);
   const [localSelectionEnd, setLocalSelectionEnd] = useState(null);
@@ -11758,6 +11759,7 @@ const App = () => {
     }
     setDraggedSectionItem({ trackId, itemType, itemId, clickOffset, multiIds });
   };
+  handleSectionItemDragStartRef.current = handleSectionItemDragStart;
 
   // ── Section / MIDI Item Resize Start ──
   const handleSectionItemResizeStart = (trackId, itemType, itemId, side, clickTime) => {
@@ -11875,7 +11877,7 @@ const App = () => {
       var dx = e.clientX - pd.startX;
       if (Math.abs(dx) > 5) {
         pendingDragRef.current = null;
-        handleSectionItemDragStart(pd.trackId, pd.itemType, pd.itemId, pd.clickOffset, true);
+        if (handleSectionItemDragStartRef.current) handleSectionItemDragStartRef.current(pd.trackId, pd.itemType, pd.itemId, pd.clickOffset, true);
       }
     };
     document.addEventListener('mousemove', handleMouseMove);