At the moment a user can only generate and download a QR Invoice Payment Part & Receipt.
In this task a new preview functionality should be added. When clicked, a "pop up" (like the existing) should open and display the payment part & receipt as PNG, 150dpi only, DIN_LANG with boundary lines and scissors symbol using liberation sans and in the same language as the web ui is used.
In case the input data is invalid, an error has to be shown.
The pop up is closable. If it is re-opened the preview image has to be recreated (input data may have been changed).