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/auth.rs

@@ -46,41 +46,12 @@ pub async fn login(
     println!("  Username: {}", request.username);
     println!("  Password length: {}", request.password.len());
 
-    // Basic validation
-    if request.username.is_empty() || request.password.is_empty() || request.server_url.is_empty() {
-        return Err(ApiError::BadRequest(
-            "Username, password, and server URL are required".to_string(),
-        ));
-    }
+    // Use the auth service login method which now handles database, sessions, and preferences
+    let response = state.auth_service.login(request).await?;
 
-    println!("✅ Input validation passed");
+    println!("✅ Login successful with session management");
 
-    // Create a token using the auth service
-    println!("📝 Created CalDAV config");
-
-    // First verify the credentials are valid by attempting to discover calendars
-    let config = CalDAVConfig::new(
-        request.server_url.clone(),
-        request.username.clone(),
-        request.password.clone(),
-    );
-    let client = CalDAVClient::new(config);
-    client
-        .discover_calendars()
-        .await
-        .map_err(|e| ApiError::Unauthorized(format!("Authentication failed: {}", e)))?;
-
-    let token = state
-        .auth_service
-        .generate_token(&request.username, &request.server_url)?;
-
-    println!("🔗 Created CalDAV client, attempting to discover calendars...");
-
-    Ok(Json(AuthResponse {
-        token,
-        username: request.username,
-        server_url: request.server_url,
-    }))
+    Ok(Json(response))
 }
 
 pub async fn verify_token(