Replaces the minimal login-style form with the full spec design:
hero section (eyebrow, headline, subtext), three labelled form sections,
2-column name grid, confirm-password field with client-side match hints,
password strength indicator, notification checkbox card, loading state on
submit, and "already have an account?" footer link.
Backend: adds notifyOnMention to RegisterRequest and wires both
notifyOnMention and notifyOnReply via updateNotificationPreferences on
invite redemption.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
d07f7debf8