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/models.rs

@@ -16,8 +16,28 @@ pub struct CalDAVLoginRequest {
 #[derive(Debug, Serialize)]
 pub struct AuthResponse {
     pub token: String,
+    pub session_token: String,
     pub username: String,
     pub server_url: String,
+    pub preferences: UserPreferencesResponse,
+}
+
+#[derive(Debug, Serialize, Deserialize, Clone)]
+pub struct UserPreferencesResponse {
+    pub calendar_selected_date: Option<String>,
+    pub calendar_time_increment: Option<i32>,
+    pub calendar_view_mode: Option<String>,
+    pub calendar_theme: Option<String>,
+    pub calendar_colors: Option<String>,
+}
+
+#[derive(Debug, Deserialize)]
+pub struct UpdatePreferencesRequest {
+    pub calendar_selected_date: Option<String>,
+    pub calendar_time_increment: Option<i32>,
+    pub calendar_view_mode: Option<String>,
+    pub calendar_theme: Option<String>,
+    pub calendar_colors: Option<String>,
 }
 
 #[derive(Debug, Serialize)]