Per argus review: the hardcoded /etc/hyperhive/gateway.htpasswd
example was wrong for operators with a custom htpasswdFile path.
Move the unauthorized.html from the static agentErrorPagesDir derivation
into a pkgs.writeText inside the lib.optionalAttrs guard where
cfg.auth.htpasswdFile is in scope and statically known non-null.
The rendered page now shows the operator's actual configured path.
ba1d096391