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 everything to Svelte 5 (deps + syntax + features)
  • first step: make it work again with Svelte5 while using partially legacy syntax and function
  • migrate everything from JS to TS (where it makes sense)
  • re-create a new Svelte5 + TS version of each component in a separate folder
  • delete old style components and make sure no errors are thrown during compilation
  • move the new Svelte5 components into the original place after all dependencies have been migrated
  • finalize: get rid of all left-over legacy functions everywhere
• 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
  • create new theming backend with DB migrations, endpoints, caching, pre-compression
  • include the new theme in the UI and always preload the stylesheets
  • create a new component to modify custom themes
  • make it possible to manually override system prefs for theming
• create a new structure for API communication in a type-safe way
• simplify the i18n setup to make it easier to understand
• improve accesibility
  • improve automatic focus and event handling in many situations
  • update all components to use proper aria labels and other things for screenreaders
• improve performance and efficiency
  • move global CSS out of inline-style in each doc into cacheable, pre-compressed external css
  • move all static HTML serving endpoints into a separate module and implement dynamic compression + caching
• prettify the UI
  • build a new Nav + SubNav for the Admin UI to make it fully responsive even down to mobile
  • improve visuals and make it prettier
• get a better logo which makes it easier to identify the Rauthy tab in browsers
• fix UI does not update when deleting attribute #672 during the migration