Do most of the theme handling from the Theme component

[matc-pub]

Description

I noticed some issues with the current way of handling themes:

• Code blocks briefly use light theme after page refresh for "browser" theme in dark mode
• "browser-compact" is not compact and always in light mode
• Live theme preview:
  • sometimes mixes light and dark elements
  • "browser" theme can be in wrong dark/light variant
  • Navigating away from dialog without saving keeps theme active
• Unrelated: AdminSettings doesn't have a theme list after page refresh

This change moves all decisions about the current theme into the Theme component. The only thing outside of the Theme component is a script embedded in the SSR which sets the data-bs-theme attribute for "browser" and "browser-compact", as the server doesn't know the correct value and waiting for all scripts to load would be to slow. The Theme and Settings components communicate using "refresh-theme" and "set-theme-override" CustomEvents.

Screenshots

Before

Code syntax highlight in wrong color scheme while loading

Live Preview

Screenshot made without saving the theme. Only "browser-compact" has problems after saving and reloading.

"browser-compact" live preview not compact

Live preview with mixed dark/light elements

Live preview "vaporwave-dark" in light

After

No screenshots. Things look as they should.

[matc-pub]

One sequence to reproduce the light/dark mixed element. With "darkly-red" saved, change to "darkly", then "browser-compat", then "browser".

Looks like "browser-compact" has to be involved.

[dessalines]

Just tested this out, and it seemed to work well, thx!