[fix] move connection cleanup off the UI thread

apps/desktop/src/gui/runtime/session_lifecycle.h

@@ -0,0 +1,58 @@
+/*
+ * @Author: DI JUNKUN
+ * @Date: 2026-09-07
+ * Copyright (c) 2026 by DI JUNKUN, All Rights Reserved.
+ */
+
+#ifndef _SESSION_LIFECYCLE_H_
+#define _SESSION_LIFECYCLE_H_
+
+#include <condition_variable>
+#include <cstddef>
+#include <mutex>
+
+namespace crossdesk::gui_detail {
+
+class SessionCallbackGate;
+
+// Decrements the count on scope exit, including early returns and exceptions.
+class CallbackGuard {
+ public:
+  explicit CallbackGuard(SessionCallbackGate& callbacks);
+  ~CallbackGuard();
+
+  CallbackGuard(const CallbackGuard&) = delete;
+  CallbackGuard& operator=(const CallbackGuard&) = delete;
+
+  explicit operator bool() const { return accepted_; }
+
+ private:
+  SessionCallbackGate& callbacks_;
+  bool accepted_;
+};
+
+// Each peer counts callbacks as they enter and leave.
+// Closing rejects new callbacks, then waits for running callbacks to finish.
+class SessionCallbackGate {
+ public:
+  bool IsActive() const;
+  CallbackGuard Enter();
+
+  // Called by the session owner, never from within this session's callbacks.
+  void Close();
+
+ private:
+  friend class CallbackGuard;
+
+  bool BeginCallback();
+  void EndCallback();
+
+  mutable std::mutex mutex_;
+  std::condition_variable callbacks_finished_;
+  bool accepting_callbacks_ = true;
+  std::size_t running_callbacks_ = 0;
+};
+
+}  // namespace crossdesk::gui_detail
+
+#endif