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/Cargo.toml

@@ -34,6 +34,10 @@ base64 = "0.21"
 thiserror = "1.0"
 lazy_static = "1.4"
 
+# Database dependencies
+sqlx = { version = "0.7", features = ["runtime-tokio", "sqlite", "uuid", "chrono", "json"] }
+tokio-rusqlite = "0.5"
+
 [dev-dependencies]
 tokio = { version = "1.0", features = ["macros", "rt"] }
 reqwest = { version = "0.11", features = ["json"] }