Refactor WebSocket server to use nlohmann::json for active connections

- Update the return type of getActiveConnections() in both websocket_server.cpp and websocket_server.h to nlohmann::json for consistency and clarity.
- Ensure proper usage of the nlohmann::json library in the WebSocket server implementation.

src/websocket_server.cpp

@@ -549,7 +549,7 @@ bool WebSocketServer::isMainAdmin(const std::string &hashedUserId) {
     return count > 0;
 }
 
-json WebSocketServer::getActiveConnections() {
+nlohmann::json WebSocketServer::getActiveConnections() {
     json result = json::array();
     
     std::shared_lock<std::shared_mutex> lock(connectionsMutex);