Show Snapmaker U1 camera in Device tab

The U1 exposes no /server/webcams/list entry;
its camera only captures after an explicit
camera.start_monitor RPC, which the Moonraker
websocket executes unauthenticated but only
answers over MQTT - so the call is fire and
forget.

Start the camera when the camera view is
shown and renew every 300 s: the printer
retires the capture task at ~362 s and
stop_monitor is accepted but ineffective,
so teardown is simply to stop renewing.

Frames land in monitor.jpg as still JPEGs
(~2 fps at interval 0), so the webview loads
a local HTML wrapper that repolls with a
cache buster.

src/slic3r/Utils/NetworkAgent.hpp

@@ -145,6 +145,7 @@ public:
     int command_ams_refresh_rfid(std::string dev_id, std::string tray_id, int sequence_id, bool lan_mode);
     int command_ams_calibrate(std::string dev_id, int ams_id, int sequence_id, bool lan_mode);
     int command_ams_select_tray(std::string dev_id, std::string tray_id, int sequence_id, bool lan_mode);
+    int command_start_camera(std::string dev_id);
     int connect_printer(std::string dev_id, std::string dev_ip, std::string username, std::string password, bool use_ssl);
     int disconnect_printer();
     int send_message_to_printer(std::string dev_id, std::string json_str, int qos, int flag);