fix: fallback channel for unconfigured tracks uses index

When midiChannel is undefined (no instrument set), compute channel
from track index to avoid defaulting to 0 and clashing with
configured tracks that use channel 0.

app/static/js/app.jsx

@@ -10407,7 +10407,15 @@ const App = () => {
     const instrumentProgram = track ? track.instrumentProgram : undefined;
     const synthEngine = track ? track.synth_engine : undefined;
     var allTracks = activeTracksRef.current || [];
-    var mainCh = track ? (track.midiChannel !== undefined ? track.midiChannel : 0) : 0;
+    var mainCh = 0;
+    if (track) {
+      if (track.midiChannel !== undefined) {
+        mainCh = track.midiChannel;
+      } else {
+        for (var mc = 0; mc < allTracks.length; mc++) { if (allTracks[mc].id === track.id) { mainCh = mc >= 9 ? mc + 1 : mc; break; } }
+        if (mainCh >= 16) mainCh = 0;
+      }
+    }
     // Compute session beat offset from target item
     var sessionBeatOffset = 0;
     var targetTrk = allTracks.find(function(t) { return t.id === st.trackId; });
@@ -10438,7 +10446,15 @@ const App = () => {
       var ghostSynth = layer.synthEngine || (ghostTrack ? ghostTrack.synth_engine : undefined);
       if (ghostProg === undefined && !ghostSynth) return;
       var ghostDest = getOrCreateTrackNode(ghostTrack, context);
-      var ghostCh = ghostTrack ? (ghostTrack.midiChannel !== undefined ? ghostTrack.midiChannel : 0) : 0;
+      var ghostCh = 0;
+      if (ghostTrack) {
+        if (ghostTrack.midiChannel !== undefined) {
+          ghostCh = ghostTrack.midiChannel;
+        } else {
+          for (var gc = 0; gc < allTracks.length; gc++) { if (allTracks[gc].id === ghostTrack.id) { ghostCh = gc >= 9 ? gc + 1 : gc; break; } }
+          if (ghostCh >= 16) ghostCh = 0;
+        }
+      }
       layer.notes.forEach(function(note) {
         var beat = (note.start_beat || 0) + sessionBeatOffset;
         var dur = note.duration_beats || 1;