Highlight buttons when using tab-key for navigation

[juliankehne]

Motivation

With #3269 most ui elements are now focusable and are being highlighted by the browser when using the tab-key for navigating the site.
Our standard buttons are not being highlighted as other elements. (Video below showing what it looks like)

Proposed Solution

Make buttons being highlighted/framed by the browser functionality when being focused while using the tab-key.

Alternatives

Leave as it is.

User Story

As a keyboard user I want to navigate ALL UI elements using the tab without losing focus so that I can work more efficient.

Additional Context

Short video that shows our buttons are not getting the frame when pressing tab:
https://github.com/user-attachments/assets/7285af2e-399f-4dee-b528-298a08d40067

Design Requirements

Not required as it is a functionality provided by a browser.

Related Issues

#3269