fix(gui): persist embedded VST GUI state across panel close/reopen

Close/reopen of Mastering panel or FX Rack unmounted the modal, losing
masterGuiOpen/fxGuiOpen state. The native bridge window stayed alive but
hidden, so reopening the panel showed no GUI and no /show was re-posted —
user could not interact with the VST GUI. Persist the embed state at module
scope so remount re-anchors and re-shows the window automatically.

app/static/js/app.jsx

@@ -12065,6 +12065,9 @@ const ExportModal = ({ open, onClose, exportSettings, setExportSettings, isExpor
     );
   };
 
+// GUI embed state survives FXRackModal unmount so the native window
+// re-shows automatically when the rack is reopened (close/reopen bug).
+let _persistFxGuiOpen = null;
 const FXRackModal = ({ track, onUpdateTrack, onClose }) => {
   const [activeType, setActiveType] = React.useState('eq');
   const [addModuleOpen, setAddModuleOpen] = React.useState(false);
@@ -12100,7 +12103,8 @@ const FXRackModal = ({ track, onUpdateTrack, onClose }) => {
   const vstChain = (track.vstFxChain || []);
   const setVstChain = (next) => { if (onUpdateTrack) onUpdateTrack(track.id, { vstFxChain: next, vstFxBypass: vstBypass }); };
   // Embedded VST GUI state (PLAN_MASTERBUS_VST_GUI_EMBED.md Phase 3)
-  const [fxGuiOpen, setFxGuiOpen] = React.useState(null); // { path, url } | null
+  const [fxGuiOpen, setFxGuiOpenState] = React.useState(_persistFxGuiOpen); // { path, url } | null
+  const setFxGuiOpen = (v) => { _persistFxGuiOpen = v; setFxGuiOpenState(v); };
   const fxGuiOpenRef = React.useRef(null);
   fxGuiOpenRef.current = fxGuiOpen;
   const closeFxGui = () => {
@@ -12624,6 +12628,9 @@ const addVst = (path) => { const fx = (fxList || []).find(f => f.path === path);
 // ──────────────────────────────────────────────
 // MASTERING MODAL COMPONENT (from md/45_MASTERING_MODULE.md)
 // ──────────────────────────────────────────────
+// GUI embed state survives MasteringModal unmount so the native window
+// re-shows automatically when the panel is reopened (close/reopen bug).
+let _persistMasterGuiOpen = null;
 const MasteringModal = ({ isOpen, onClose, masteringSettings, setMasteringSettings }) => {
   const ozState = masteringSettings;
   const setOzState = setMasteringSettings;
@@ -12639,7 +12646,8 @@ const MasteringModal = ({ isOpen, onClose, masteringSettings, setMasteringSettin
   // ── VST FX masterbus (native_bridge --render-fx, áp dụng khi Export/Offline render) ──
   const masterVstChain = (ozState.vstFxChain || []);
   // Embedded VST GUI state (PLAN_MASTERBUS_VST_GUI_EMBED.md Phase 2)
-  const [masterGuiOpen, setMasterGuiOpen] = React.useState(null); // { path, url } | null
+  const [masterGuiOpen, setMasterGuiOpenState] = React.useState(_persistMasterGuiOpen); // { path, url } | null
+  const setMasterGuiOpen = (v) => { _persistMasterGuiOpen = v; setMasterGuiOpenState(v); };
   const [activeMasterVstPath, setActiveMasterVstPath] = React.useState(null);
   const masterGuiOpenRef = React.useRef(null);
   masterGuiOpenRef.current = masterGuiOpen;