frontend: ability to run under a different base path

[RobertRosca]

A requirement for #24 is being able to have the frontend work even when it is configured to run on a different base path, e.g. /feat/foo instead of /.

Played around with this a bit and got it kind of working, but not all the way. Changes I think are required are:

1. Some configuration to set the base (e.g. env var)
2. Vite should read this and set the correct base path - https://vitejs.dev/guide/build#public-base-path
3. React router should be aware of it as well - https://reactrouter.com/en/main/routers/create-browser-router#optsbasename
4. Calls in js should also either be aware of it or always use relative addresses (e.g. stuff like history.navigate("/home") and fetch("/oauth/logout", should reference the base path or be relative where possible) - looks like this can be handled by vite by using ${import.meta.env.BASE_URL}/... for urls.