[feat] add online status indicators for recent connections

src/gui/device_presence.h

@@ -0,0 +1,36 @@
+/*
+ * @Author: DI JUNKUN
+ * @Date: 2026-02-28
+ * Copyright (c) 2026 by DI JUNKUN, All Rights Reserved.
+ */
+
+#ifndef _DEVICE_PRESENCE_H_
+#define _DEVICE_PRESENCE_H_
+
+#include <mutex>
+#include <string>
+#include <unordered_map>
+
+class DevicePresence {
+ 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_);
+    return cache_.count(device_id) > 0 && cache_.at(device_id);
+  }
+
+  void Clear() {
+    std::lock_guard<std::mutex> lock(mutex_);
+    cache_.clear();
+  }
+
+ private:
+  std::unordered_map<std::string, bool> cache_;
+  mutable std::mutex mutex_;
+};
+
+#endif