Implement diary web application with YubiKey WebAuthn authentication

Implement diary web application with YubiKey WebAuthn authentication

- Create main application entry point (index.php) with mobile-friendly UI
- Implement action handling according to prompt specifications:
  - action('start') called automatically after login
  - action('new') for creating triplets
  - action('box text') for showing message boxes
  - action('input help') for text input
  - action('set name') for setting values
  - action('choose keyword') for editing triplets by keyword
  - action('edit ID') for editing specific triplets
  - Default triplet shown when none exist
  - Triplet search by keyword
- Implement user registration with YubiKey WebAuthn (register.php)
- Implement user login with YubiKey WebAuthn
- Update config.php with robust database initialization
- Update Database.php and WebAuthnManager.php classes
- Add DEPLOY.md deployment guide
- Clean up unnecessary files

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>