[enhancement] Support for Configurable Base Path in WebAdmin Interface

[Sofahamster]

Hi,

I would like to propose an enhancement to add support for a configurable base path (or base URL) for the WebAdmin interface. This feature would make it easier to deploy the interface behind a reverse proxy, such as Nginx, when serving it on a subfolder (subpath) rather than the root URL.

For example:
If the WebAdmin interface is served at https://domain.tld/stalwartadmin/, the application should be able to handle the /stalwartadmin/ prefix in URLs seamlessly.

The nginx configuration would probably look like this:

location /stalwartadmin/ {
  proxy_pass https://localhost:8080/;
  proxy_set_header Host $host;
  proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
  proxy_set_header X-Real-IP $remote_addr;
  proxy_set_header X-Forwarded-Proto $scheme;
}

Many modern web interfaces allow working behind reverse proxies on a sub-path, for example gitea allows setting the ROOT_URL, and the rspamd web interface even works on a sub-path without any configuration.