fix: default instrument for tracks without instrument set

_playNoteFluid: when no synthEngine and no program, fallback to
program 0 on channel (default piano) so unconfigured tracks still
produce sound via FluidSynth

app/static/js/services/soundfontPlayer.js

@@ -392,6 +392,11 @@
                 try {
                     _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) {}
             }
             if (ch === undefined) ch = (usedBank === 128 ? 9 : 0);
             var ctx = getCtx();