fix(ui): black screen — isExportingRef.current = isExporting dat truoc khai bao isExporting (TDZ ReferenceError moi render); bo sync, bounce set true/false tuong minh

app/static/js/app.jsx

@@ -17852,8 +17852,9 @@ const App = () => {
   isPlayingRef.current = isPlaying;
   // Bounce realtime = project đang phát thật; guardPlay (scheduleMidiNoteDispatch)
   // phải cho note fire dù isPlayingRef bị render ghi đè =false khi bounce setState.
+  // (Không sync từ state ở đây — isExporting khai báo sau; bounce set true/false
+  // tường minh ở triggerBounceExport/finally.)
   const isExportingRef = useRef(false);
-  isExportingRef.current = isExporting;
   const [selectionStart, setSelectionStart] = useState(null);
   const [selectionEnd, setSelectionEnd] = useState(null);
   const [selectionFollowsTempo, setSelectionFollowsTempo] = useState(true);