The app allows you to either select one of the built-in card numbers that I have found on the internet. (Only for demonstration purposes, of course!) Or you can just enter a code manually and create the loyalty card from it. The link to a card can be shared (e.g. if you and your SO use the same loyalty card).
At the moment, it is very basic and only has one single design that is supposed to visually mimic one of the biggest loyalty card providers that I know.
Architectural-wise, the app is written in TypeScript, powered by React and Vite. The app does not transmit and/or store your codes anywhere but on your device; it’s a single-page application that runs entirely in your browser, without any server component.
Adding new card designs and/or preset codes can be achieved quite easily, so feel free to fork and contribute!