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.cpp

@@ -788,6 +788,13 @@ int NetworkAgent::command_ams_select_tray(std::string dev_id, std::string tray_i
     return -1;
 }
 
+int NetworkAgent::command_start_camera(std::string dev_id)
+{
+    if (m_printer_agent)
+        return m_printer_agent->command_start_camera(dev_id);
+    return -1;
+}
+
 int NetworkAgent::connect_printer(std::string dev_id, std::string dev_ip, std::string username, std::string password, bool use_ssl)
 {
     if (m_printer_agent)