fix: section tab clears midiItems, breaks ghost+dropdown

handleEditSectionInTab overwrote midiItems: [] when cloning
section.tracks, stripping all MIDI data from section context.
PianoRollTabEditor uses activeTracks which resolves section
tracks -- midiItems must be preserved for dropdown switcher,
ghost note extraction, and instrument settings to work.

app/static/js/app.jsx

@@ -8497,7 +8497,17 @@ const App = () => {
     if (existing) { setActiveTab(existing.id); showToast(`Tab "${section.name}" already open.`, 'info'); return; }
     const tabId = 'session_' + Date.now();
     const tabName = section.name || 'Section';
-    const clonedTracks = section.tracks ? JSON.parse(JSON.stringify(section.tracks)) : tracks.filter(t => t.id === trackId).map(t => ({
+    const clonedTracks = section.tracks ? (JSON.parse(JSON.stringify(section.tracks))).map(t => ({
+      ...t,
+      clips: [],
+      sections: [],
+      markers: [],
+      isArmed: false,
+      monitoringEnabled: true,
+      instrumentId: t.instrumentId || null,
+      instrumentProgram: t.instrumentProgram,
+      instrumentName: t.instrumentName || null
+    })) : tracks.filter(t => t.id === trackId).map(t => ({
       ...t,
       clips: [],
       sections: [],