refactor vpn settings

fix: nekoray_core freedom udp loopback

feat: copy custom config

ui/mainwindow.h

@@ -70,6 +70,8 @@ private slots:
 
     void on_menu_routing_settings_triggered();
 
+    void on_menu_vpn_settings_triggered();
+
     void on_menu_hotkey_settings_triggered();
 
     void on_menu_add_from_input_triggered();