fix: range loop selection maintains bar count on BPM change

- Add prevBpmRef to track previous BPM
- BPM effect recalculates selectionStart/End using bar count
- startBar = oldStart / oldSecondsPerBar
- newStart = startBar * newSecondsPerBar (same for end)

app/static/js/app.jsx

@@ -6402,12 +6402,25 @@ const App = () => {
   }]);
   const [appWarningModal, setAppWarningModal] = useState(null);
   const [bpm, setBpm] = useState(localStorage.getItem('studio_bpm') || '120');
-  const [draggedClip, setDraggedClip] = useState(null); // { trackId, clickOffset, buffer, name, volume, color }
+  const prevBpmRef = useRef(bpm);
+  const [draggedClip, setDraggedClip] = useState(null);
   const [hoveredTrackId, setHoveredTrackId] = useState(null);
-  // Recalculate item/section durations when BPM changes (maintain bar count)
+  // Recalculate item/section/selection durations when BPM changes
   useEffect(() => {
+    const oldSpb = prevBpmRef.current ? (60.0 / parseFloat(prevBpmRef.current)) * 4 : null;
     const bpmVal = parseFloat(bpm) || 120;
     const secondsPerBar = (60.0 / bpmVal) * 4;
+    // Recalculate range loop selection to maintain bar count
+    if (oldSpb && selectionStart !== null && selectionEnd !== null && selectionEnd > selectionStart) {
+      const startBar = selectionStart / oldSpb;
+      const endBar = selectionEnd / oldSpb;
+      if (endBar - startBar > 0.01) {
+        setSelectionStart(startBar * secondsPerBar);
+        setSelectionEnd(endBar * secondsPerBar);
+      }
+    }
+    prevBpmRef.current = bpm;
+    // Recalculate item/section durations
     updateActiveTracks(prev => prev.map(t => ({
       ...t,
       midiItems: (t.midiItems || []).map(m => {