fix: change AppHeaderButton style fixing #7208

[camdenvaughan]

Summary

Fixes #7208 by changing style of AppHeaderButton to use :focus-visible instead of :focus. This was noticed when closing the media library the "Media" header link would remain blue until anything else was interacted with.

Test plan

Passed all tests after running npm run test. Everything still functions identically when navigating the site with a keyboard as well as a mouse.

Checklist

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