fix: Synth button shown on section/audio tracks

Synth button was unconditional in TCP. Now gated:
track.midiItems?.length > 0 && !track.sections?.length
Section tracks: Synth hidden (sections are containers).
MIDI tracks: Synth shown.
Audio tracks: Synth hidden.
FX button stays unconditional for all track types.

app/static/js/app.jsx

@@ -16492,7 +16492,7 @@ const App = () => {
         className: "w-3 h-3"
       })), " FX: ", /*#__PURE__*/React.createElement("span", {
         className: "text-zinc-500 font-normal"
-      }, track.fxType || "None")), /*#__PURE__*/React.createElement("button", {
+      }, track.fxType || "None")), (track.midiItems && track.midiItems.length > 0 && (!track.sections || track.sections.length === 0)) && /*#__PURE__*/React.createElement("button", {
         onClick: (e) => { e.stopPropagation(); openInstrumentSelector(track.id); },
         className: "px-2.5 py-1 bg-zinc-800 hover:bg-zinc-700 text-amber-400 border border-amber-800 rounded text-xs font-bold flex items-center gap-1 max-w-[120px]"
       }, /*#__PURE__*/React.createElement("span", {