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/SnapmakerPrinterAgent.hpp

@@ -16,6 +16,10 @@ public:
     AgentInfo        get_agent_info() override { return get_agent_info_static(); }
 
     bool fetch_filament_info(std::string dev_id) override;
+    int command_start_camera(std::string dev_id) override;
+
+protected:
+    std::string webcam_stream_override(const std::string& base_url) const override;
 
 private:
     // Combine filament_type + filament_sub_type into a unified type string