fix(piano-roll): loop only on btn + instant stop

- Sub-tab selection loop only active when st.isLooping is true
- End-of-playback loop uses st.isLooping, not global isLoopingSelection
- SonicSF.stopAll() sets gain 0 and stops oscillators immediately

app/static/js/app.jsx

@@ -9529,7 +9529,7 @@ const App = () => {
       const bufferPos = startBufferOffsetRef.current + elapsed * speedFactor;
 
       // Loop sub-tab selection (bufferPos is buffer-time)
-      if (st.selectionStart !== null && st.selectionEnd !== null && st.selectionStart !== st.selectionEnd && (st.isLooping || isLoopingSelection)) {
+      if (st.selectionStart !== null && st.selectionEnd !== null && st.selectionStart !== st.selectionEnd && st.isLooping) {
         const start = Math.min(st.selectionStart, st.selectionEnd);
         const end = Math.max(st.selectionStart, st.selectionEnd);
         if (bufferPos >= end) {
@@ -9559,7 +9559,7 @@ const App = () => {
       })() : st.buffer.duration;
       if (bufferPos >= effectiveDuration) {
         stopAllPlayback();
-        if (isLoopingSelection) {
+        if (st.isLooping) {
           setSubTabs(prev => prev.map(s => s.id === activeTabRef.current ? {
             ...s,
             currentTime: 0,