feat: redesign plain layout pages

[JammingBen]

Redesign the plain layout pages and reduce the flicker after logging in and out and when resolving private links by:

• matching the initial loading spinner and the AppLoadingSpinner.
• matching the background colors of the initial page render and the loaded theme.
• introducing a new bare layout that only renders the AppLoadingSpinner to avoid rendering some text like You are being logged in for a split second.

Also, make using dark: prefixes on Tailwind classes work by correctly setting document.documentElement.dataset.colorScheme.

[JammingBen]

@kulmann I'm a bit undecided on the You are being logged in part. It previously appeared directly after login for a split second (depending on the network). But it doesn't really add much value and just states the obvious (if the user is fast enough to read it in the first place). So I just expanded the loading spinner a bit longer instead. What do you think?

[kulmann]

@kulmann I'm a bit undecided on the You are being logged in part. It previously appeared directly after login for a split second (depending on the network). But it doesn't really add much value and just states the obvious (if the user is fast enough to read it in the first place). So I just expanded the loading spinner a bit longer instead. What do you think?

Yes, much better. Having a text that is only briefly there might cause the typical confusion of "I was not fast enough to read it, what was that? did something bad happen?". loading spinner is sufficient. 👍

[kulmann]

really nice improvement 🤓