Svelte 5 Migration 1

[sebadob]

This is the first PR in a probably long series of smaller ones. The end goal of all PRs is:

• 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

#642