Implement lightweight auth system with SQLite

Added SQLite database for session management and user preferences storage,
allowing users to have consistent settings across different sessions and devices.

Backend changes:
- Added SQLite database with users, sessions, and preferences tables
- Implemented session-based authentication alongside JWT tokens
- Created preference storage/retrieval API endpoints
- Database migrations for schema setup
- Session validation and cleanup functionality

Frontend changes:
- Added "Remember server" and "Remember username" checkboxes to login
- Created preferences service for syncing settings with backend
- Updated auth flow to handle session tokens and preferences
- Store remembered values in LocalStorage (not database) for convenience

Key features:
- User preferences persist across sessions and devices
- CalDAV passwords never stored, only passed through
- Sessions expire after 24 hours
- Remember checkboxes only affect local browser storage

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/src/handlers.rs

@@ -2,9 +2,11 @@
 mod auth;
 mod calendar;
 mod events;
+mod preferences;
 mod series;
 
 pub use auth::{get_user_info, login, verify_token};
 pub use calendar::{create_calendar, delete_calendar};
 pub use events::{create_event, delete_event, get_calendar_events, refresh_event, update_event};
+pub use preferences::{get_preferences, logout, update_preferences};
 pub use series::{create_event_series, delete_event_series, update_event_series};