Fix register page Turnstile validation

[gaidheal1]

Summary

• Explicitly render the Cloudflare Turnstile widget when the register page mounts so SPA navigation reliably initializes the security check
• Wire the terms checkbox through the shared form component correctly so required validation uses checked state for checkboxes
• Add focused frontend tests covering the register flow and checkbox validation

Validation (from original PR #271)

• Vitest: Input, Form, RegisterPage tests
• Frontend build check