Fix header icons focus and hover style for better accessibility (#6409)

## Why are these changes needed?

This PR improves keyboard accessibility by ensuring that header
links/icons have visual feedback (underline and color change) on both
hover and keyboard focus states. Also adds smooth scaling animation.

## Related issue number

Related issue: #6090 

## Checks

- [☑️] I've included any doc changes needed for
<https://microsoft.github.io/autogen/>. See
<https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md> to
build and test documentation locally.
- [☑️] I've added tests (if relevant) corresponding to the changes
introduced in this PR.
- [☑️] I've made sure all auto checks have passed.

Co-authored-by: peterychang <49209570+peterychang@users.noreply.github.com>