Show existing files for exisiting themes

[Arnei]

Fixes the last complaint from #915

After creating the theme, uploaded files are not listed. In the old UI, these files were visible, and clicking on them would prompt the browser to download them.

How to test this

In order to test this you will need to enable themes in Opencast. This can be done in etc/org.opencastproject.organization-mh_default_org.cfg configuration file, which should enable a new main menu option.

[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-1528

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-1528

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/1528/2026-03-17_10-14-03/ .
It might take a few minutes for it to become available.