fix: AI-generated tracks save synth_engine + pass to applyAITrackInstrument

- Build synth_engine object with type, plugin_id, soundfont_bank,
  soundfont_program, soundfont_id
- Save to targetTrack.synth_engine so schedulePianoRollMidi reads it
- Pass synthEngine as 3rd arg to applyAITrackInstrument (was missing)
- Remove redundant separate loadSoundFont call (applyAITrackInstrument
  chains to selectInstrument which already calls loadSoundFont)

app/static/js/app.jsx

@@ -13873,11 +13873,17 @@ const App = () => {
             if (aiTrack.soundfont_bank !== undefined && aiTrack.soundfont_program !== undefined) {
               targetTrack.soundfont_bank = aiTrack.soundfont_bank;
               targetTrack.soundfont_program = aiTrack.soundfont_program;
+              targetTrack.soundfont_id = aiTrack.soundfont_id || '';
+              const sfEngine = {
+                type: 'soundfont',
+                plugin_id: aiTrack.soundfont_id ? 'sf_' + aiTrack.soundfont_id : null,
+                soundfont_bank: aiTrack.soundfont_bank,
+                soundfont_program: aiTrack.soundfont_program,
+                soundfont_id: aiTrack.soundfont_id || ''
+              };
+              targetTrack.synth_engine = sfEngine;
               if (window.SonicSF && window.SonicSF.applyAITrackInstrument) {
-                window.SonicSF.applyAITrackInstrument(aiTrack.soundfont_bank, aiTrack.soundfont_program);
-              }
-              if (window.SonicSF && window.SonicSF.loadSoundFont && aiTrack.soundfont_id) {
-                window.SonicSF.loadSoundFont(aiTrack.soundfont_id);
+                window.SonicSF.applyAITrackInstrument(aiTrack.soundfont_bank, aiTrack.soundfont_program, sfEngine);
               }
             }
           });