feat: block free email provider signups

Mirrors the existing client-side check in the v3 signup form
(company-email-validator via Vuelidate) so non-business emails can no
longer slip in via Google OAuth, direct API calls, or any other path
that bypassed the form.

The deny list ships as config/deny_listed_email_domains.yml — picked up
automatically by ValidEmail2 — and is trimmed to the 6.5k consumer-only
domains the gem's disposable list doesn't already cover.

lib/custom_exceptions/account.rb

@@ -7,6 +7,8 @@ module CustomExceptions::Account
         I18n.t 'errors.signup.blocked_domain'
       elsif @data[:disposable]
         I18n.t 'errors.signup.disposable_email'
+      elsif @data[:free_email_provider]
+        I18n.t 'errors.signup.free_email_provider'
       elsif !@data[:valid]
         I18n.t 'errors.signup.invalid_email'
       end