Fix: Make Add User dialog a multi-step wizard

[priya-tik]

This PR refactors the "Add User" dialog to behave like a multi-step wizard, improving the user experience by splitting user details and role assignment into separate tabs. This change ensures better usability and mirrors the design of similar multi-step forms in the interface.

Note:
Please let me know if any additional changes or improvements are needed!

[github-actions]

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[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-1197

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-1197

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/1197/2025-05-21_16-52-15/ .
It might take a few minutes for it to become available.

[github-actions]

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

[JamesUoM]

Though the Next button functions as expected it doesn't change the look& feel to that of the Wizard Model i.e steps instead of tabs.

[gregorydlogan]

This also doesn't look like a multi-step wizard. Compare creating an event to creating a user:

• It's not obvious that there are two steps right away cf the new event modal's five dots right at the top
• It's not obvious that modifying the roles is pretty much required and that it must happen after username selection etc
• It's possible to go straight to the roles without inputting the required parts

[Arnei]

This changes exactly what the text in #1195 requests and as such technically solves the issue. But as others mentioned, this is not enough to solve the spirit of the issue which requires the "Add User" dialog to behave and look like all the other "Add" and "Create" dialogs.

I would argue that solving the spirit of the issue is what we really want here. Just technically solving the issue does not lead to the improved user experience and usability we want, but just causes different kinds of confusion for our users. Therefore I would not merge this PR as it is right now.

If you would like to make the "Add User" dialog behave and feel like other "Add dialogs", I would suggest looking at the "Add group" dialog and going from there. Others have already given pointers to what the key differences are.