Username signup rules in `gnoweb` UI

[Kouteki]

Description

We need a straightforward username registration flow, available in gnoweb with some guardrails on what the registration system accepts as usernames.

Acceptance Criteria

• Allow users to register usernames with a simple, intuitive flow in the gnoweb frontend
• Validate the username input:
  • Alphabetic characters only (lowercase Latin)
  • Disallow usernames starting with g1
  • Disallow usernames starting with gl
• Return an error if the username input is invalid

[Kouteki]

User signup flow should somehow collect the email, even if it's a form outside gnoweb