[refactor] organize GUI code by responsibility

src/gui/runtime/device_presence_cache.h

@@ -0,0 +1,41 @@
+/*
+ * @Author: DI JUNKUN
+ * @Date: 2026-02-28
+ * Copyright (c) 2026 by DI JUNKUN, All Rights Reserved.
+ */
+
+#ifndef CROSSDESK_GUI_DEVICE_PRESENCE_CACHE_H_
+#define CROSSDESK_GUI_DEVICE_PRESENCE_CACHE_H_
+
+#include <mutex>
+#include <string>
+#include <unordered_map>
+
+namespace crossdesk {
+
+class DevicePresenceCache {
+public:
+  void SetOnline(const std::string &device_id, bool online) {
+    std::lock_guard<std::mutex> lock(mutex_);
+    cache_[device_id] = online;
+  }
+
+  bool IsOnline(const std::string &device_id) const {
+    std::lock_guard<std::mutex> lock(mutex_);
+    const auto it = cache_.find(device_id);
+    return it != cache_.end() && it->second;
+  }
+
+  void Clear() {
+    std::lock_guard<std::mutex> lock(mutex_);
+    cache_.clear();
+  }
+
+private:
+  std::unordered_map<std::string, bool> cache_;
+  mutable std::mutex mutex_;
+};
+
+} // namespace crossdesk
+
+#endif // CROSSDESK_GUI_DEVICE_PRESENCE_CACHE_H_