fix: _playNoteFluid else branch leaked other track's SF program 0

When a track has no instrument configured, _playNoteFluid called
program_change(ch, 0). FluidSynth searches ALL loaded SoundFonts
to resolve program 0 — if only another track's SoundFont is loaded,
that SF's program 0 gets applied to the wrong track.

Fix: use oscillator fallback directly instead of program_change.

app/static/js/services/soundfontPlayer.js

@@ -393,10 +393,13 @@
                     _fluidModule._fluid_synth_program_change(_synthPtr, ch, usedProg);
                 } catch (e) {}
             } else {
-                if (ch === undefined) ch = 0;
-                try {
-                    _fluidModule._fluid_synth_program_change(_synthPtr, ch, 0);
-                } catch (e) {}
+                // No instrument configured: use oscillator fallback directly.
+                // Do NOT call program_change(ch, 0) — FluidSynth would pick
+                // program 0 from whatever SoundFont is loaded (could be
+                // another track's SF), causing cross-track instrument leak.
+                var ctx = getCtx();
+                self._playNoteFallback(note, velocity, durationMs, startTime, undefined, null, channel, undefined);
+                return;
             }
             if (ch === undefined) ch = (usedBank === 128 ? 9 : 0);
             var ctx = getCtx();