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/services/soundfontPlayer.js

@@ -170,17 +170,14 @@
                 try {
                     if (entry.gain) {
                         entry.gain.gain.cancelScheduledValues(now);
-                        entry.gain.gain.setValueAtTime(entry.gain.gain.value || 0.8, now);
-                        entry.gain.gain.linearRampToValueAtTime(0, now + 0.02);
+                        entry.gain.gain.setValueAtTime(0, now);
                     }
                     if (entry.osc) {
-                        try { entry.osc.stop(now + 0.025); } catch (e) { }
+                        try { entry.osc.stop(now); } catch (e) { }
                     }
                 } catch (e) { }
             });
-            setTimeout(() => {
-                Object.keys(activeOscillators).forEach(k => delete activeOscillators[k]);
-            }, 50);
+            Object.keys(activeOscillators).forEach(k => delete activeOscillators[k]);
         },
 
         // Save user SoundFont to IndexedDB via window.SonicStorage