Initial setup form allows "." in initial admin username

Reproduction steps:

• Start a fresh wiki without demo data
• On the first screen to setup first user type a login with a "." (e.g. "test.-.user.com")
• Provide password and validate the form

Expected result:

• The form cannot be validate and an error should explain that the login is invalid
• No user profile is created

Obtained result:

• The form is validated but the page is refreshed to display it again
• The console log displays a warning:

  2026-03-12 08:51:30,873 [qtp1926096844-50 - http://localhost:8080/xwiki/bin/distribution/XWiki/Distribution?xredirect=%2Fxwiki%2Fbin%2Fview%2FMain%2F] WARN  nticationFailureLoggerListener - Authentication failure with login [test.-.user.com]
  

• a user is created with the wrong login

Original description: See https://forum.xwiki.org/t/8322 - according to tmortagne the initial setup screen should not allow creation of an initial admin username that contains the "." character. Creating issue here as requested.