Show the print-failure snapshot in the device error dialog

When a print fails, DeviceErrorDialog now fetches the printer's captured camera
frame of the failure and shows it in place of the generic HMS illustration,
falling back to the local image and a drawn placeholder on older plugins or errors.

- Agent: add get_hms_snapshot through NetworkAgent / IPrinterAgent (BBL calls the
  bound plugin symbol; other agents no-op, so old plugins degrade gracefully).
- DeviceManager: parse and clear m_print_error_img_id from the print-error message.
- Dialog: tiered cloud/local/placeholder image reusing the single image widget,
  with a liveness-guarded async callback decoded on the UI thread.

src/slic3r/Utils/IPrinterAgent.hpp

@@ -139,6 +139,12 @@ public:
      */
     virtual int request_bind_ticket(std::string* ticket) = 0;
 
+    /**
+     * Fetch the cloud snapshot image captured at a print failure.
+     * Returns 0 if the request was dispatched; the image body arrives via callback(body, http_status).
+     */
+    virtual int get_hms_snapshot(std::string dev_id, std::string file_name, std::function<void(std::string, int)> callback) = 0;
+
     /**
      * Register callback for fatal HTTP errors.
      */