Add example code club theme to avoid future regressions

[jamdelion]

See slack convo here: https://raspberrypifoundation.slack.com/archives/C02JBAA2NFP/p1781251204953259

I think we don't have a good way to test theming changes which will make incompatibilities like this inevitable.
If we had an example theme set up in the editor ui repo that we could easily switch to (and maybe even a visual regression test) that might help us catch these issues.

Ideally the example would appear here: https://staging-editor-static.raspberrypi.org/branches/main/web-component.html