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", {

wiki.md

@@ -539,6 +539,12 @@
 - **Ghi chú/Test (nếu có):** `npm run build` pass. Ctrl+Click+Drag section/MIDI item → copy đến vị trí mới, item không bị selected.
 ---
 
+### [2026-07-28 08:36] Task: Synth chỉ hiển thị cho MIDI track, không cho section/audio track
+- **Tóm tắt thay đổi:** Synth button trong TCP giờ conditional: chỉ hiển thị khi track có midiItems (>0) và KHÔNG có sections. FX button giữ nguyên cho mọi track type. Logic đúng: Synth áp dụng cho MIDI item, FX áp dụng cho section/MIDI/audio clip item.
+- **Các file ảnh hưởng:** `app/static/js/app.jsx`, `app/static/js/app.precompiled.js`
+- **Ghi chú/Test (nếu có):** `npm run build` pass. Track có sections → ẩn Synth. Track MIDI → hiện Synth. Track audio → ẩn Synth. FX hiện ở mọi track.
+---
+
 ### [2026-07-28 08:31] Task: Section track auto-load soundfont 1/prog 0 từ MAIN qua `...t` spread
 - **Tóm tắt thay đổi:** `...t` trong fallback branch spread `soundfont_id`, `soundfont_bank`, `soundfont_program`, `instrument_source` từ parent MAIN track. Dù đã null `instrumentProgram` và `synth_engine`, các field `soundfont_*` vẫn khiến FluidSynth load soundfont khi play. Fix: override tất cả soundfont fields thành null/undefined.
 - **Các file ảnh hưởng:** `app/static/js/app.jsx`, `app/static/js/app.precompiled.js`