feat: embed native VST GUI in mastering + fx rack panels (bridge --fx-gui frame capture + input relay)

Linux parts (0L1-4L1 per TASKS_LINUX_WINDOWS.md):
- native_bridge: FxGuiServer.cpp HTTP server (Winsock) — /frame BitBlt +
  PrintWindow fallback -> JPEG (libjpeg-turbo q70), /input PostMessage relay
  (mouse/wheel/key, coords scaled), /close, /ping; RenderFxJob fxGuiSetup +
  run_fx_gui_server (offscreen); main.cpp --fx-gui dispatch; vcpkg +
  CMakeLists WIN32-only + JPEG.
- api: open_fx_gui embed flag spawn --fx-gui, parse SF_FXGUI_PORT -> embed_url,
  fx-gui/close endpoint; 4 unit tests.
- ui: VstGuiEmbed component (frame refresh 100ms, ping 2s, input relay,
  scale coords) shared by MasteringModal + FXRackModal; swap graph<->GUI,
  cleanup on unmount, dead-bridge toast fallback.
- docs: TASKS_LINUX_WINDOWS.md, WALKTHROUGH_WINDOWS.md, PLAN status.
Windows tasks 0W1-4W1 remain (build + E2E real VST).

app/api/v1/plugins.py

@@ -503,6 +503,14 @@ class FxGuiRequest(BaseModel):
     """Mở native GUI của VST FX qua bridge (Windows desktop)."""
     path: str
     name: Optional[str] = None
+    # embed=True → bridge --fx-gui (off-screen + HTTP frame/input server,
+    # PLAN_MASTERBUS_VST_GUI_EMBED.md); embed=False → cửa sổ rời cũ.
+    embed: bool = False
+
+
+class FxGuiCloseRequest(BaseModel):
+    """Đóng bridge fx-gui đã mở cho plugin (embed mode)."""
+    path: str
 
 
 class PreviewRequest(BaseModel):
@@ -717,12 +725,41 @@ async def open_fx_gui(req: FxGuiRequest, current_user: dict = Depends(get_curren
     except Exception:
         os.close(fd)
         raise HTTPException(status_code=500, detail="Không ghi được job file cho bridge.")
-    cmd = [exe, "--open-fx-gui", job_path]
+    cmd = [exe, "--fx-gui" if req.embed else "--open-fx-gui", job_path]
     try:
+        if req.embed:
+            proc = subprocess.Popen(cmd, stdout=subprocess.PIPE,
+                                    stderr=subprocess.STDOUT, text=True,
+                                    close_fds=os.name != "nt")
+            # Bridge prints SF_FXGUI_PORT=<port> after binding its HTTP server.
+            embed_url = None
+            try:
+                for line in proc.stdout:
+                    line = (line or "").strip()
+                    if line.startswith("SF_FXGUI_PORT="):
+                        port = int(line.split("=", 1)[1])
+                        embed_url = f"http://127.0.0.1:{port}"
+                        break
+            except Exception:
+                pass
+            if not embed_url:
+                proc.terminate()
+                try:
+                    proc.wait(timeout=5)
+                except Exception:
+                    proc.kill()
+                raise HTTPException(status_code=500, detail="Bridge không báo port fx-gui (mở GUI thất bại).")
+            proc._sf_plugin_path = plugin_path
+            proc._sf_embed_url = embed_url
+            _register_fx_gui_process(proc)
+            return {"success": True, "started": True, "already_running": False,
+                    "embed_url": embed_url, "cmd": cmd}
         proc = subprocess.Popen(cmd, close_fds=os.name != "nt")
         proc._sf_plugin_path = plugin_path
         _register_fx_gui_process(proc)
         return {"success": True, "started": True, "already_running": False, "cmd": cmd}
+    except HTTPException:
+        raise
     except Exception as e:
         try:
             os.remove(job_path)
@@ -731,6 +768,26 @@ async def open_fx_gui(req: FxGuiRequest, current_user: dict = Depends(get_curren
         raise HTTPException(status_code=500, detail=f"Không mở được GUI của VST: {e}")
 
 
+@router.post("/fx-gui/close")
+async def close_fx_gui(req: FxGuiCloseRequest, current_user: dict = Depends(get_current_user)):
+    """Đóng bridge fx-gui đang chạy cho plugin (embed mode) — frontend gọi khi
+    user đóng panel embed. Không lỗi nếu plugin chưa mở (closed=False)."""
+    enforce_password_changed(current_user)
+    plugin_path = (req.path or "").strip()
+    if not plugin_path:
+        raise HTTPException(status_code=400, detail="Thiếu đường dẫn VST plugin.")
+    for proc in _FX_GUI_PROCESSES:
+        if proc is not None and proc.poll() is None and getattr(proc, "_sf_plugin_path", None) == plugin_path:
+            proc.terminate()
+            try:
+                proc.wait(timeout=5)
+            except Exception:
+                proc.kill()
+            _prune_fx_gui_processes()
+            return {"success": True, "closed": True}
+    return {"success": True, "closed": False}
+
+
 _FX_GUI_PROCESSES = []  # list[subprocess.Popen] — các bridge fx-gui do app spawn
 
 def _register_fx_gui_process(proc):