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.

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/1600/2026-05-04_05-29-07/ .
It might take a few minutes for it to become available.