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/1197/2025-05-07_08-32-39/ .
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.