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>

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 {