feat: track cloud sign_up event in dataLayer

Fires the sign_up dataLayer event already read by GTM for the three
signup paths on the cloud version: email/password (register.tsx),
GitHub/Google OAuth (via better-auth's newUserCallbackURL, distinct
from callbackURL so returning-user logins aren't counted), and
invitation acceptance. All gated behind isCloud, same as the rest of
the analytics wiring.