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.precompiled.js

@@ -1140,7 +1140,9 @@ const[selectedTrackId,setSelectedTrackId]=useState('1');const[selectedItemIds,se
 // khi hết âm).
 const isPlayingRef=useRef(false);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.
-const isExportingRef=useRef(false);isExportingRef.current=isExporting;const[selectionStart,setSelectionStart]=useState(null);const[selectionEnd,setSelectionEnd]=useState(null);const[selectionFollowsTempo,setSelectionFollowsTempo]=useState(true);const selectionRef=useRef({start:null,end:null});selectionRef.current={start:selectionStart,end:selectionEnd};const[selectionMode,setSelectionMode]=useState(null);// 'global' (from ruler) | 'local' (from track)
+// (Không sync từ state ở đây — isExporting khai báo sau; bounce set true/false
+// tường minh ở triggerBounceExport/finally.)
+const isExportingRef=useRef(false);const[selectionStart,setSelectionStart]=useState(null);const[selectionEnd,setSelectionEnd]=useState(null);const[selectionFollowsTempo,setSelectionFollowsTempo]=useState(true);const selectionRef=useRef({start:null,end:null});selectionRef.current={start:selectionStart,end:selectionEnd};const[selectionMode,setSelectionMode]=useState(null);// 'global' (from ruler) | 'local' (from track)
 const[sweepSelect,setSweepSelect]=useState(null);const isSweepingRef=useRef(false);const sweepStartRef=useRef(0);const sweepTrackIdRef=useRef(null);const sweepStartYRef=useRef(0);const sweepEndYRef=useRef(0);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);const[zoom,setZoom]=useState(()=>{// Persist zoom qua reload (user yêu cầu giữ kích thước items sau refresh)
 try{const v=parseFloat(localStorage.getItem('sf_zoom'));if(isFinite(v)&&v>0)return v;}catch(e){}return 100;});React.useEffect(()=>{try{localStorage.setItem('sf_zoom',String(zoom));}catch(e){}// Zoom TIMELINE (App) — serialize project dùng cái này (window.__currentZoom