feat(auth): add public registration for the first account

[ibourgeois]

Goal\nAllow a new user to create the first account in the system so the admin bootstrap path has something to work with.\n\n## Scope\n- Re-enable Fortify registration\n- Add a custom registration page using the shared design system\n- Keep the flow narrow and aligned with the current name/email model\n\n## Out of Scope\n- Invitations\n- Organization onboarding\n- Role assignment during registration\n\n## Acceptance Criteria\n- A guest can register a new account\n- The new account can immediately authenticate\n- The registration page uses the shared auth shell and UI components\n\n## Dependencies\n- Depends on #7, #9\n\n## Test Plan\n- Add feature tests for registration page response and submission\n- Verify the created user can log in afterward