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/IPrinterAgent.hpp

@@ -93,6 +93,11 @@ public:
     { return ORCA_NETWORK_ERR_CMD_NOT_SUPPORTED; }
     virtual int command_ams_select_tray(std::string, std::string, int, bool)
     { return ORCA_NETWORK_ERR_CMD_NOT_SUPPORTED; }
+    // why: some printers emit camera frames only while explicitly asked, and retire the
+    // capture task on their own - the camera view starts it and renews it. Printers with an
+    // always-on stream need nothing here, hence the honest refusal by default.
+    virtual int command_start_camera(std::string)
+    { return ORCA_NETWORK_ERR_CMD_NOT_SUPPORTED; }
 
     /**
      * Establish a direct LAN connection to a printer.