Rework login flow

[andreaskienle]

Brought up by @maximilianbraun

Todo

• Evaluate if we even need a login screen or if we could redirect immediately to the IdP (differences between HSP vs. standalone version?) (also consider the token exchange flow on HSP ([SPIKE] HSP: Evaluate Support for HSP Auth Token #175))
• Check how to handle errors when the token we get from the IdP does not provide sufficient authorizations (are the error messages clear and helpful?)

If we keep the login screen, we should fix the dark mode: