Support IPv6 via webui

[DerDanilo]

Currently it doesn't seem possible to set the IPv6 via the webui. Shouldn't the core check for proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for; and then display whatever is present there?

The DDNS server also has an IPv6 and I set a sub-sub domain that only has the IPv6 IP. Connections works fine but the webui refused to show the clients IPv6. I tested some other headers in the nginx proxy. Still only the IPv4.
This shouldn't even be possible since the client then only connects with IPv6. Is there some sort of caching for this? This should be disabled.

How can we solve this?