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,

frontend/src/components/sidebar.rs

@@ -81,12 +81,6 @@ impl Style {
         }
     }
 
-    pub fn display_name(&self) -> &'static str {
-        match self {
-            Style::Default => "Default",
-            Style::Google => "Google Calendar",
-        }
-    }
 
     pub fn stylesheet_path(&self) -> Option<&'static str> {
         match self {

frontend/src/services/mod.rs

@@ -2,4 +2,3 @@ pub mod calendar_service;
 pub mod preferences;
 
 pub use calendar_service::CalendarService;
-pub use preferences::PreferencesService;

frontend/src/services/preferences.rs

@@ -15,6 +15,7 @@ pub struct UserPreferences {
 }
 
 #[derive(Debug, Serialize)]
+#[allow(dead_code)]
 pub struct UpdatePreferencesRequest {
     pub calendar_selected_date: Option<String>,
     pub calendar_time_increment: Option<i32>,
@@ -23,10 +24,12 @@ pub struct UpdatePreferencesRequest {
     pub calendar_colors: Option<String>,
 }
 
+#[allow(dead_code)]
 pub struct PreferencesService {
     base_url: String,
 }
 
+#[allow(dead_code)]
 impl PreferencesService {
     pub fn new() -> Self {
         let base_url = option_env!("BACKEND_API_URL")
@@ -147,7 +150,7 @@ impl PreferencesService {
         let session_token = LocalStorage::get::<String>("session_token")
             .map_err(|_| "No session token found".to_string())?;
         
-        let mut request = UpdatePreferencesRequest {
+        let request = UpdatePreferencesRequest {
             calendar_selected_date: LocalStorage::get::<String>("calendar_selected_date").ok(),
             calendar_time_increment: LocalStorage::get::<u32>("calendar_time_increment").ok().map(|i| i as i32),
             calendar_view_mode: LocalStorage::get::<String>("calendar_view_mode").ok(),