Fix input element not displaying file in Create Event

[Arnei]

Fixes #946.

If you open the create event modal and upload a file in the source tab, switch to a different tab and then switch back to the source tab, the name of the uploaded file will not be displayed, even though the file is actually uploaded.
While this is just a visual bug, it can be very confusing for users.

This patch fixes the issue by adding some html and css to fake the look of an input dialog, instead of relying on the native implementation. This gives us full control of what the user sees. Afaik it is also necessary, because browsers will not allow feeding an exisiting file handle into a file
upload input.

How to test this

Can be tested as is. Experiment with uploading and removing files, and moving between different tabs.

[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-1529

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-1529

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/1529/2026-03-17_13-50-15/ .
It might take a few minutes for it to become available.