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.cpp

@@ -1792,6 +1792,14 @@ int MachineObject::command_ams_refresh_rfid2(int ams_id,  int slot_id)
     return this->publish_json(j);
 }
 
+int MachineObject::command_start_camera()
+{
+    if (!m_agent) return -1;
+    // why: this fires from the camera view's renew timer, so a refusal must stay silent -
+    // show_unsupported_dlg() here would pop a dialog every ~5 min on every other printer.
+    return m_agent->command_start_camera(get_dev_id());
+}
+
 
 int MachineObject::command_ams_select_tray(std::string tray_id)
 {