fix: PIANO_ROLL activeTracks resolves main tracks, not section

When PIANO_ROLL sub-tab is opened from SECTION-TAB,
activeTab switches to midi_xxx, so activeTracks fell
back to main project tracks. Section trackIds don't
exist in main tracks -> dropdown empty, ghost layers
empty, instrument data wrong.
Fix: check subTabs for PIANO_ROLL type, resolve parent
section tab from parent_tab_id, return its tracks.

app/static/js/app.jsx

@@ -7315,9 +7315,15 @@ const App = () => {
     }
   };
   const activeTracks = useMemo(() => {
-    const st = sessionTabs.find(s => s.id === activeTab);
-    return st ? st.tracks : tracks;
-  }, [activeTab, sessionTabs, tracks]);
+    var st = sessionTabs.find(s => s.id === activeTab);
+    if (st) return st.tracks;
+    var pr = subTabs.find(s => s.id === activeTab && s.type === 'PIANO_ROLL');
+    if (pr && pr.parent_tab_id && pr.parent_tab_id.startsWith('session_')) {
+      var parentSt = sessionTabs.find(s => s.id === pr.parent_tab_id);
+      if (parentSt) return parentSt.tracks;
+    }
+    return tracks;
+  }, [activeTab, sessionTabs, subTabs, tracks]);
   const activeTracksRef = useRef([]);
   activeTracksRef.current = activeTracks;
   const sessionTabsRef = useRef(sessionTabs);

app/static/js/app.precompiled.js

@@ -216,7 +216,7 @@ buffer:track.buffer,startTime:track.startTime||0,clips:track.clips?track.clips.m
 const[activeTab,setActiveTab]=useState('main');const[subTabSelectedNodeTime,setSubTabSelectedNodeTime]=useState(null);const[subTabNormVal,setSubTabNormVal]=useState(0);const[subTabGainVal,setSubTabGainVal]=useState(100);const[subTabPitchVal,setSubTabPitchVal]=useState(0);const[subTabs,setSubTabs]=useState([]);// [{id, label, trackId, startTime, endTime, buffer, effects, currentTime, selectionStart, selectionEnd, isPlaying}, ...]
 const[sessionTabs,setSessionTabs]=useState([]);// [{id, name, tracks}, ...]
 const[tabContextMenu,setTabContextMenu]=useState(null);// { x, y, tabId, tabType }
-const handleSetTabColor=(tabId,tabType,color)=>{if(tabType==='session'){setSessionTabs(prev=>prev.map(s=>s.id===tabId?{...s,color:color}:s));const tab=sessionTabs.find(s=>s.id===tabId);if(tab){setTracks(prev=>prev.map(t=>{if(!t.sections||t.sections.length===0)return t;return{...t,sections:t.sections.map(s=>{if(s.sectionId===tab.sectionId){return{...s,color:color};}return s;})};}));}}else{setSubTabs(prev=>prev.map(s=>s.id===tabId?{...s,color:color}:s));}};const activeTracks=useMemo(()=>{const st=sessionTabs.find(s=>s.id===activeTab);return st?st.tracks:tracks;},[activeTab,sessionTabs,tracks]);const activeTracksRef=useRef([]);activeTracksRef.current=activeTracks;const sessionTabsRef=useRef(sessionTabs);sessionTabsRef.current=sessionTabs;const[selectionCleared,setSelectionCleared]=useState(false);// LOOP_EDITOR_2.md §4.2
+const handleSetTabColor=(tabId,tabType,color)=>{if(tabType==='session'){setSessionTabs(prev=>prev.map(s=>s.id===tabId?{...s,color:color}:s));const tab=sessionTabs.find(s=>s.id===tabId);if(tab){setTracks(prev=>prev.map(t=>{if(!t.sections||t.sections.length===0)return t;return{...t,sections:t.sections.map(s=>{if(s.sectionId===tab.sectionId){return{...s,color:color};}return s;})};}));}}else{setSubTabs(prev=>prev.map(s=>s.id===tabId?{...s,color:color}:s));}};const activeTracks=useMemo(()=>{var st=sessionTabs.find(s=>s.id===activeTab);if(st)return st.tracks;var pr=subTabs.find(s=>s.id===activeTab&&s.type==='PIANO_ROLL');if(pr&&pr.parent_tab_id&&pr.parent_tab_id.startsWith('session_')){var parentSt=sessionTabs.find(s=>s.id===pr.parent_tab_id);if(parentSt)return parentSt.tracks;}return tracks;},[activeTab,sessionTabs,subTabs,tracks]);const activeTracksRef=useRef([]);activeTracksRef.current=activeTracks;const sessionTabsRef=useRef(sessionTabs);sessionTabsRef.current=sessionTabs;const[selectionCleared,setSelectionCleared]=useState(false);// LOOP_EDITOR_2.md §4.2
 // ── Temp Edit Tab (LOOP_EDITOR_2.md §1.2 Sub Tab) ──
 const[tempTabActive,setTempTabActive]=useState(false);const[tempTabBuffer,setTempTabBuffer]=useState(null);const[tempTabTrackId,setTempTabTrackId]=useState(null);const[tempTabOrigStart,setTempTabOrigStart]=useState(0);const[tempTabOrigEnd,setTempTabOrigEnd]=useState(0);const tempTabCanvasRef=useRef(null);// Effect parameters for temp tab
 const[tempTabEffects,setTempTabEffects]=useState({reverse:false,gainDb:0,fadeInMs:0,fadeOutMs:0});// ── Auth / User State ──