isAdminHost() only matched localhost and the configured server_ip
(set to the public IP by the install script). Installations accessed
via RFC 1918 addresses (10.x, 192.168.x, 172.16-31.x) fell through
to the redirect-domain handler and returned 'Domain nicht konfiguriert'.
Extract isPrivateOrLoopbackIp() to recognize all private and loopback
addresses as admin hosts. No security risk: redirect domains are never
private IPs.
