fix: maintain item duration in bars when BPM changes

- Add length_bars field to midiItems and sections at creation
- useEffect on bpm recalculates duration = length_bars * secondsPerBar
- Fix section creation: use secondsPerBar instead of hardcoded 4s
- Handle existing items without length_bars (no-op)

app/static/js/app.jsx

@@ -6165,6 +6165,7 @@ const deserializeTracksList = (schemaTracks, secondsPerBar, sectionStore) => {
           name: item.name,
           startTime: item.start_bar * secondsPerBar,
           duration: item.duration_bars * secondsPerBar,
+          length_bars: item.duration_bars || 4,
           notes: (src.notes || []).map(n => ({
             id: n.id,
             pitch: n.pitch || 60,
@@ -6183,6 +6184,7 @@ const deserializeTracksList = (schemaTracks, secondsPerBar, sectionStore) => {
           name: item.name,
           start: item.start_bar * secondsPerBar,
           duration: item.duration_bars * secondsPerBar,
+          length_bars: item.duration_bars || 4,
           sectionId: secId,
           tracks: secContainer ? deserializeTracksList(secContainer.tracks, secondsPerBar, sectionStore) : null
         });
@@ -6414,6 +6416,16 @@ const App = () => {
   const [bpm, setBpm] = useState(localStorage.getItem('studio_bpm') || '120');
   const [draggedClip, setDraggedClip] = useState(null); // { trackId, clickOffset, buffer, name, volume, color }
   const [hoveredTrackId, setHoveredTrackId] = useState(null);
+  // Recalculate item/section durations when BPM changes (maintain bar count)
+  useEffect(() => {
+    const bpmVal = parseFloat(bpm) || 120;
+    const secondsPerBar = (60.0 / bpmVal) * 4;
+    updateActiveTracks(prev => prev.map(t => ({
+      ...t,
+      midiItems: (t.midiItems || []).map(m => m.length_bars ? { ...m, duration: m.length_bars * secondsPerBar } : m),
+      sections: (t.sections || []).map(s => s.length_bars ? { ...s, duration: s.length_bars * secondsPerBar } : s)
+    })));
+  }, [bpm]);
   const openPanel = id => {
     if (id === 'export') setShowExportPanel(true); else if (id === 'ai') setShowAIPanel(true); else if (id === 'python_tools') setShowPythonToolsPanel(true); else if (id === 'selection') setShowSelectionPanel(true);
   };
@@ -10594,6 +10606,7 @@ const App = () => {
             name: 'Recorded MIDI',
             startTime: recordingStartTimeRef.current,
             duration: Math.ceil(totalDurationBeats / 4) * secondsPerBar,
+            length_bars: Math.ceil(totalDurationBeats / 4),
             notes: recordedNotes
         };
         return {
@@ -10613,6 +10626,7 @@ const App = () => {
             name: 'Recorded MIDI',
             startTime: recordingStartTimeRef.current,
             duration: Math.ceil(totalDurationBeats / 4) * secondsPerBar,
+            length_bars: Math.ceil(totalDurationBeats / 4),
             notes: recordedNotes
         };
         updateActiveTracks(prev => prev.map(t => {
@@ -11860,11 +11874,13 @@ const App = () => {
     const curTracks = activeTracks;
     const track = curTracks.find(t => t.id === selectedTrackId);
     if (!track) { showToast('Chọn track trước', 'warning'); return; }
+    const secondsPerBar = (60.0 / (parseInt(bpm) || 120)) * 4;
     const section = {
       id: `sec_${Date.now()}`,
       name: 'Section',
       start: currentTime,
-      duration: 4,
+      duration: 4 * secondsPerBar,
+      length_bars: 4,
       color: track.color || '#06b6d4'
     };
     updateActiveTracks(prev => prev.map(t => t.id === selectedTrackId ? {
@@ -11885,6 +11901,7 @@ const App = () => {
       name: 'MIDI Item',
       startTime: currentTime,
       duration: 4 * secondsPerBar,
+      length_bars: 4,
       notes: [],
       color: '#a78bfa'
     };
@@ -13861,6 +13878,7 @@ const App = () => {
               name: `${composition_title || 'AI Theme'} - ${aiTrack.track_name}`,
               startTime: itemStartTimeSec,
               duration: durationSec,
+              length_bars: total_bars,
               notes: aiTrack.notes.map((note, index) => ({
                 id: `note_ai_${Date.now()}_${index}`,
                 pitch: note.pitch,