Show registration modal only when unregistered

[gregorydlogan]

This PR only shows the registraiton modal when the adopter has not already registered.

This PR contains a few extra bits that haven't been turned on yet - notably the warning bell stuff for when the core can't reach the registration server.

This PR now uses the bell if the user is not registered. Notably:
If you haven't registered at all:

If you have registered, but haven't agreed to the latest ToU:

Clicking on either of these notices opens the registration modal.

Requires opencast/opencast#7786 on the backend.

TODOs:

• Make the new strings into translatable strings rather than hardcoding them.
• In the case of an out of date ToU, uncheck the agreed-to-ToU checkbox when presenting the registration modal, and highlight what's wrong.
• In the case of an out of date ToU, launch the registration modal to the input pane, not the general info pane.

Fixed #1516

[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/admin-interface:pr-1600

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/admin-interface:pr-1600

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/1600/2026-05-11_21-10-31/ .
It might take a few minutes for it to become available.

[gregorydlogan]

Now that the GHA issues are done, I made a final change and I think this is ready for re-review @Arnei

[gregorydlogan]

Ok, so I think this is more or less done...

I noticed that the registration modal itself was using a script in utils, so I removed it and moved its functionality into the selector. Hopefully I didn't miss anything obvious. This now requires opencast/opencast#7786 as well.