fix: multi-item MOVE also discarded cross-track updates

Same bug as copy: return statement only updated
targetTrackId/drag.trackId, discarding items from other tracks.
Fix: always return { sections: midSections, midiItems: midMidis }
for ALL tracks in the map (arrays are sliced per-track).

app/static/js/app.jsx

@@ -11832,9 +11832,7 @@ const App = () => {
                 if (idx >= 0) midMidis[idx] = { ...midMidis[idx], startTime: Math.max(0, newVal) };
               }
             });
-            return t.id === targetTrackId
-              ? { ...t, sections: midSections, midiItems: midMidis }
-              : { ...t, sections: (t.id === drag.trackId ? midSections : t.sections), midiItems: (t.id === drag.trackId ? midMidis : t.midiItems) };
+            return { ...t, sections: midSections, midiItems: midMidis };
           }
           const items = drag.itemType === 'section' ? (t.sections || []) : (t.midiItems || []);
           const updatedItems = items.filter(it => it.id !== drag.itemId);