CustomLoginHandler._render missing **kwargs of newer LoginHandler._render

### CustomLoginHandler._render missing **kwargs of newer LoginHandler._render

Hi, sorry this is not a proper PR. I don't use git often and I'm customizing my Jupyter installation via runtime overrides in the python config file instead of editing the firstuseauthenticator source.

I noticed the arguments mismatch with respect to the newer login.py base class [1] which was causing my customizations to crash. I managed to work around it, but adding **kwargs may fix it properly. The kwargs seems to have been added to LoginHandler._render on the commit of Mar 22, 2025 and was absent in commit of Feb 6 2024 [3] and earlier.

Thanks!

1. https://github.com/jupyterhub/jupyterhub/blob/main/jupyterhub/handlers/login.py
2. https://github.com/jupyterhub/jupyterhub/blob/ef568e3d614c7facab1d2429cfd56b0f4b17a7b6/jupyterhub/handlers/login.py
3. https://github.com/jupyterhub/jupyterhub/blob/1839a2cc1cd87e1f9c09d0a7b619b3653068b0ff/jupyterhub/handlers/login.py

Locate CustomLoginHandler._render in the firstuseauthenticator repository and compare its arguments with the referenced JupyterHub handlers/login.py implementation. Update the override to handle the newer arguments, then run the repository's relevant tests or authentication checks to confirm custom login rendering no longer crashes.

Written by the indexing model from the issue text.