[Improved, Fixed] Signup security hotfix

Improvements

• The sign-up form now blocks disposable or alias-forwarding email addresses (SimpleLogin, Proton Pass, addy.io, etc.) and clearly explains why, helping keep your team’s account secure.
• Domain checks now include common subdomains (e.g., mail.yahoo.com) to ensure personal email addresses are detected more reliably.

Fixes

• Rejected Google OAuth sign-ups now display a clear error message that directs users to contact support instead of failing silently.