fix: auto-compute length_bars for legacy items on BPM change

- Items without length_bars: compute bars from duration/secondsPerBar
- Round to nearest quarter bar for musical accuracy
- Recalculate duration = bars * new secondsPerBar for all items
- Maintains correct bar count for both legacy and new items

app/static/js/app.jsx

@@ -6422,8 +6422,23 @@ const App = () => {
     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)
+      midiItems: (t.midiItems || []).map(m => {
+        if (m.length_bars) return { ...m, duration: m.length_bars * secondsPerBar };
+        if (m.duration) {
+          // Legacy item without length_bars: compute bars from current duration/BPM
+          const bars = Math.max(0.25, Math.round(m.duration / secondsPerBar * 4) / 4);
+          return { ...m, length_bars: bars, duration: bars * secondsPerBar };
+        }
+        return m;
+      }),
+      sections: (t.sections || []).map(s => {
+        if (s.length_bars) return { ...s, duration: s.length_bars * secondsPerBar };
+        if (s.duration) {
+          const bars = Math.max(0.25, Math.round(s.duration / secondsPerBar * 4) / 4);
+          return { ...s, length_bars: bars, duration: bars * secondsPerBar };
+        }
+        return s;
+      })
     })));
   }, [bpm]);
   const openPanel = id => {