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/GUI/DeviceManager.hpp

@@ -812,6 +812,7 @@ public:
     int command_ams_select_tray(std::string tray_id);
     int command_ams_refresh_rfid(std::string tray_id);
     int command_ams_refresh_rfid2(int ams_id, int slot_id);
+    int command_start_camera();
     int command_ams_control(std::string action);
     int command_ams_drying_stop();
     int command_start_extrusion_cali(int tray_index, int nozzle_temp, int bed_temp, float max_volumetric_speed, std::string setting_id = "");