fix: brush draw preview note uses wrong instrument

Line 5273 called playNote with undefined program + no channel,
defaulting to channel 0 with last-set instrument (may be Track 2's).
Fix: resolve parent track from st.trackId, pass correct channel
and synth_engine/instrumentProgram.

app/static/js/app.jsx

@@ -5270,7 +5270,9 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, activeTracks, onClos
       // Play the note with SoundFont
       if (window.SonicSF) {
         const ctx = getAudioContext();
-        window.SonicSF.playNote(pitch, 0.8, 300, ctx.currentTime, undefined, null);
+        var dwTrk = activeTracks.find(function(t) { return t.id === st.trackId; });
+        var dwCh = window.SonicPianoRoll ? window.SonicPianoRoll.getTrackMidiChannel(dwTrk, activeTracks) : (dwTrk ? dwTrk.midiChannel : 0);
+        window.SonicSF.playNote(pitch, 0.8, 300, ctx.currentTime, dwTrk ? dwTrk.instrumentProgram : undefined, null, dwCh, dwTrk ? dwTrk.synth_engine : undefined);
       }
     }
   };