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/GUI/DeviceManager.hpp

@@ -417,6 +417,7 @@ public:
     bool    is_system_printing();
 
     int     print_error;
+    std::string m_print_error_img_id;
     static std::string get_error_code_str(int error_code);
     std::string get_print_error_str() const { return MachineObject::get_error_code_str(this->print_error); }