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>

frontend/styles.css

@@ -289,6 +289,27 @@ body {
     cursor: not-allowed;
 }
 
+.remember-checkbox {
+    display: flex;
+    align-items: center;
+    gap: 0.5rem;
+    margin-top: 0.5rem;
+}
+
+.remember-checkbox input[type="checkbox"] {
+    width: auto;
+    margin: 0;
+    cursor: pointer;
+}
+
+.remember-checkbox label {
+    margin: 0;
+    font-size: 0.875rem;
+    color: #666;
+    cursor: pointer;
+    user-select: none;
+}
+
 .login-button, .register-button {
     width: 100%;
     padding: 0.75rem;