enh(a11y): User management settings dialog

[Pytal]

• For [BITV] 9.2.4.7/8.1 - Button "Settings" - Since the focus highlighting is solely accomplished through a color change, the contrast ratio between the focused and unfocused state should be at least 3:1, but currently it is only 1.1:1. (1) #37017

Summary

	Screenshot
Before
After

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[JuliaKirschenheuter]

Thank you @Pytal for extracting modal elements to an extra component and cleaning up!
Unfortunately contrast issue for default theme is still there:

[szaimen]

@JuliaKirschenheuter where is the contrast issue here? I think it looks fine? Can xou explain in words?

[susnux]

If it is about the focus state of the settings button, then it should be fixed on the nextcloud-vue library, as this does not introduce any custom styling there.

[kesselb]

Very nice 👍

[szaimen]

Tested and works but didnt review the code

[Pytal]

Referring to the hover state in the second image @JuliaKirschenheuter? As a11y only concerns keyboard focus then I'd say that should be addressed separately with the design team

[Pytal]

Feel free to suggest any followups @JuliaKirschenheuter :)