Fix nested buttons in dropdown warning

[Arnei]

Fixes #1156.

Fixes a console warning for the table size dropdown and the bottom right of most pages. Also slightly fixes the CSS
to avoid overlapping of elements.

Warning this fixes:

Before:

After:

How to test this

Can be tested as usual.

[github-actions]

Use docker or podman to test this pull request locally.

Run test server using develop.opencast.org as backend:

podman run --rm -it -p 127.0.0.1:3000:3000 ghcr.io/opencast/opencast-admin-interface:pr-1160

Specify a different backend like stable.opencast.org:

podman run --rm -it -p 127.0.0.1:3000:3000 -e PROXY_TARGET=https://stable.opencast.org ghcr.io/opencast/opencast-admin-interface:pr-1160

It may take a few seconds for the interface to spin up.
It will then be available at http://127.0.0.1:3000.
For more options you can pass on to the proxy, take a look at the README.md.

[github-actions]

This pull request is deployed at test.admin-interface.opencast.org/1160/2025-03-20_12-50-05/ .
It might take a few minutes for it to become available.

[NadiaUoM]

Verify all the relevant pages working as expected.