Fix compiler warnings

- Remove unused import in auth handler
- Remove unused PreferencesService export
- Fix unused mutable variable in preferences
- Remove unused display_name method from Style enum
- Add dead_code attributes for future preferences service

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

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

backend/src/handlers/auth.rs

@@ -2,7 +2,6 @@ use axum::{extract::State, http::HeaderMap, response::Json};
 use std::sync::Arc;
 
 use crate::calendar::CalDAVClient;
-use crate::config::CalDAVConfig;
 use crate::{
     models::{ApiError, AuthResponse, CalDAVLoginRequest, CalendarInfo, UserInfo},
     AppState,