- Add ALTCHA service with challenge generation and verification
- Update auth and admin handlers to use ALTCHA tokens
- Replace Turnstile widget with ALTCHA widget in Flutter app
- Update admin frontend to use ALTCHA token
- Add ALTCHA challenge endpoints for both auth and admin
- Maintain development bypass for testing
- Remove Turnstile dependencies from authentication flow
- Add development bypass to Flutter sign_in_screen.dart
- Re-enable Turnstile in admin_handler.go with dev bypass
- Add development bypass to admin login page
- Resolves login issues during development/testing
