UI Migration + Prettify

[sebadob]

This issue will keep track of the UI migration to Svelte 5 and making it prettier at the same time.

The current UI setup is a few years old and has some issues regarding DX and maintenance, as well as some CSS problems, accessibility is missing in lots of places, does not provide light / dark mode (tricky in combination with client branding), and uses a pretty complicated setup for i18n.

All of this will be solved and the work will be split into multiple PRs:

• migrate to Svelte 5 (deps + syntax + features everywhere)
• change the way i18n is done from backend -> frontend and pre-compile as much as possible
• change the way theming / branding is done and provide light + dark mode possibilities - the tricky party here is the branding of the login page which is individual for each client
• migrate everything from JS to TS (where it makes sense)
• update all components to provide better accessibility
• improve automatic focus and event handling in many situations
• improve visuals and make it prettier
• get a better logo which makes it easier to identify the Rauthy tab in browsers