fix: section track inherits soundfont_id/bank/program via ...t spread

...t in fallback branch copied soundfont_id, soundfont_bank,
soundfont_program, instrument_source from parent MAIN track
even though instrumentProgram and synth_engine were nulled.
FluidSynth picks up these fields and auto-loads the SoundFont.
Fix: override all soundfont/synth fields to null/undefined.

app/static/js/app.jsx

@@ -8517,6 +8517,10 @@ const App = () => {
       instrumentId: null,
       instrumentProgram: undefined,
       instrumentName: null,
+      soundfont_id: null,
+      soundfont_bank: undefined,
+      soundfont_program: undefined,
+      instrument_source: null,
       synth_engine: undefined
     }));
     setSessionTabs(prev => [...prev, { id: tabId, name: tabName, sectionId: sectionId, tracks: clonedTracks, color: section.color || null }]);