feat(auth): improve hosted registration experience

[maartenbode]

Summary

• add hosted-plan context to the registration page while keeping self-hosted registration unchanged
• simplify password entry without changing server-side confirmation validation
• add optional Google Analytics hooks for registration views, methods, and completed signups
• preserve signup attribution for new GitHub and GitLab users

Configuration

Hosted deployments can set GOOGLE_ANALYTICS_ID to enable analytics. Self-hosted installations remain unaffected by default.

Testing

• npm run format:check
• npm run types
• npm run build
• php artisan test tests/Feature/Auth/RegistrationTest.php tests/Feature/Auth/GitHubAuthenticationTest.php tests/Feature/Auth/GitLabAuthenticationTest.php

43 tests passed with 163 assertions.