[Feature] Auth Route Guards & Onboarding Navigation Logic #115

Implement client-side and layout route guards to enforce authentication and guide new users through onboarding.

• Create useRequireAuth hook to protect routes like /dashboard, /create, /forms/*, /templates/create.
• Create AuthGuard layout wrapper for protected route segments.
• Implement navigation logic: After OAuth login, inspect user profile state (username, fullName). If incomplete, redirect to /onboarding; otherwise redirect to /dashboard.
• Handle automatic session expiration, 401 response handling, and clear logout cleanup.