Add warning for localhost subdomain limitations in farm mode

[cgalo5758]

Warn about localhost subdomain issues in farm mode

Authentication breaks when using localhost subdomains like test.localhost in farm mode. Works fine on localhost, fails on test.localhost with confusing errors.

The problem: browsers can't share cookies between localhost and subdomains due to RFC 6761 restrictions. Same setup works great with *.localtest.me domains.

This adds a simple warning when someone tries to use localhost subdomains:

WARNING: Localhost subdomains may cause authentication issues in farm mode.
  Consider using *.localtest.me domains for reliable local development.
  See: https://tools.ietf.org/html/rfc6761#section-6.3

Saves developers time debugging authentication failures and points them to a working solution.