Fix and improve diary-web application

Fix and improve diary-web application

- Fix vendor autoload path in WebAuthnManager.php
- Fix default triplet label to match prompt requirements
- Store user settings in database instead of session only
- Add searchTripletsByKeyword and getTripletById methods to TripletManager
- Add FOREIGN KEY constraint for users.yubikey_id
- Use IF NOT EXISTS for all table creation and index creation
- Improve error handling in database initialization

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