Fix 3 bug standalone: VKB chiếm phím V, TCP FX button mở FX chain, VST GUI embed qua bridge

- VKB: stopImmediatePropagation thay stopPropagation (keydown+keyup) — chặn
  transport hotkey 'v' mở prompt Gain khi VKB đang mở (VKB đăng ký trước).
- TCP FX button: onClick -> window.__openFxRack(track.id, track.name), title
  'Mở Track FX Chain', giữ caret ▾ (title 'FX nhanh (cũ)') cho dropdown legacy.
- VST FX GUI embed: copy fx_vst_bridge.exe vào bundle (engine.spec binaries),
  find_bridge_exe tìm được trong _internal/ — hết lỗi 409 'Không tìm thấy bridge'.

Verified bằng Playwright: VKB không mở Gain prompt, FX chain rack mở + GUI VST
embed hiển thị (cả track FX rack lẫn Master FX Chain).

app/static/js/app.jsx

@@ -17449,12 +17449,17 @@ const App = () => {
       if (e.type === 'keydown') {
         if (e.repeat) return;
         e.preventDefault();
-        e.stopPropagation();
+        // stopImmediatePropagation (không chỉ stopPropagation): chặn CÁC
+        // handler capture khác CÙNG window (transport hotkeys ~20193 — phím v
+        // = Gain, s = split, ...) — stopPropagation chỉ chặn propagation tới
+        // element khác, không chặn listener cùng node cùng phase (bug user:
+        // VKB mở, phím V vẫn bật prompt Gain).
+        e.stopImmediatePropagation();
         const vel = Math.max(1, Math.min(127, parseInt(s.velocity) || 100)) / 127;
         if (vkbNoteOnRef.current) vkbNoteOnRef.current(pitch, vel);
       } else {
         e.preventDefault();
-        e.stopPropagation();
+        e.stopImmediatePropagation();
         if (vkbNoteOffRef.current) vkbNoteOffRef.current(pitch);
       }
     };
@@ -31723,7 +31728,8 @@ STRICT CONSTRAINTS:
         "data-lucide": "upload",
         className: "w-3 h-3"
       })), " File"), /*#__PURE__*/React.createElement("button", {
-        onClick: e => { e.stopPropagation(); setFxSelectorTrackId(track.id); },
+        onClick: e => { e.stopPropagation(); window.__openFxRack(track.id, track.name); },
+        title: "Mở Track FX Chain",
         className: "px-2.5 py-1 bg-zinc-800 hover:bg-zinc-700 text-purple-400 border border-purple-900 rounded text-xs font-bold flex items-center gap-1"
       }, /*#__PURE__*/React.createElement("span", {
         className: "inline-flex items-center shrink-0"
@@ -31732,7 +31738,11 @@ STRICT CONSTRAINTS:
         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"), /*#__PURE__*/React.createElement("span", {
+        title: "FX nhanh (cũ)",
+        className: "ml-0.5 text-zinc-500 hover:text-purple-300 cursor-pointer",
+        onClick: e => { e.stopPropagation(); setFxSelectorTrackId(track.id); }
+      }, "▾")), /*#__PURE__*/React.createElement("button", {
         onClick: (e) => { e.stopPropagation(); openInstrumentSelector(track.id); },
         className: "px-1.5 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-[90px] shrink-0"
       }, /*#__PURE__*/React.createElement("span", {