fix(a11y): show post action items when focus is within the post

[davwheat]

Changes proposed in this pull request:

Shows post action items when the currently focused element is within the parent .Post.

Previously, tabbing through Flarum would result in focus being "lost" on the screen, since the post actions were invisible, even when focused. This prevents this issue.

Reviewers should focus on:

I considered only checking is focus was within the .Post-actions section, but decided that showing the actions whenever an element within the post is focused would closer match the existing hover-based functionality.

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.