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/NetworkAgent.cpp

@@ -937,4 +937,11 @@ int NetworkAgent::request_bind_ticket(std::string* ticket)
     return -1;
 }
 
+int NetworkAgent::get_hms_snapshot(std::string dev_id, std::string file_name, std::function<void(std::string, int)> callback)
+{
+    if (m_printer_agent)
+        return m_printer_agent->get_hms_snapshot(dev_id, file_name, callback);
+    return -1;
+}
+
 } // namespace Slic3r