chore: minor admin frontend cleanup

[DavideIadeluca]

Companion to flarum/framework#4188

Fixes #0000

Changes proposed in this pull request:

• Remove unused classname to match structure of core, ensuring the new Button inside the dropdown is left aligned
• Increase type safety
• Add a11y titles
• Ditch username() helper inside translations (reason being [object Object] is shown in the translation when in use, did also a brief dive into the codebase and didn't find any other occasion where the username() helper was used inside a translation).
• Refactor translations to better match structure of core

Reviewers should focus on:

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.

Required changes:

• Related documentation PR: (Remove if irrelevant)