Reduce duplicate code in main page

[Arnei]

Adds a single main page component, so pages don't have to import the different headers and footer again and again. Also adds a table page component, since almost all of our pages are just a big table.

Can be tested as usual, should result in no functional changes.

[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-1369

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-1369

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/1369/2025-07-10_14-55-34/ .
It might take a few minutes for it to become available.

[gregorydlogan]

~Doesn't build, likely from changes in #1347 ~

This was caused by #1371, just ignore me...

[github-actions]

This pull request has conflicts ☹
Please resolve those so we can review the pull request.
Thanks.