`+choices` focus option

1. Add `focus` option to code blocks choices (e.g. `choice=Hono[focus]`) => the dropdown is expended like we had earlier (but only for *this* code block, avoiding the ugly overlap we had).
1. Also, maybe some kind of visual hint that draws the user's attention? Maybe some kind of colored outline around the dropdown?
1. Maybe we can hide the dropdowns again (like we had before)? But when one of the +choices is `focus` => show *all* dropdowns. I think we can do it, but let's first focus on the other two points.

This is experimental, let's see if we can get something nice 👀

> [!NOTE]
> The syntax `choice=Hono[focus]` is unfortunate because `focus` applies to the choice group, not the choice itself — can a better syntax be found?

See also:
- https://github.com/brillout/docpress/pull/132#issuecomment-4440150469
- https://github.com/brillout/docpress/issues/91
- https://github.com/brillout/docpress/issues/166

Start by locating the TypeScript implementation for code-block choices and their dropdown behavior, then review the linked discussion in pull request 132 and issues 91 and 166. Done should include a focus option for one code block, a visual attention hint, and the agreed dropdown behavior, with the syntax decision resolved.

Written by the indexing model from the issue text.