feat(register): redesign register page to match spec

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>

frontend/.gitignore

@@ -34,3 +34,8 @@ src/lib/paraglide_bak*
 
 # Playwright auth state — regenerated at the start of each CI run via auth.setup.ts
 e2e/.auth/
+
+**/test-results/**
+
+# Proofshot browser verification artifacts
+proofshot-artifacts/