Add unit tests for core components and layouts

- **ThemeToggle:** Introduce comprehensive tests to validate button functionality, dropdown options, and active theme indicators.
- **ThemeProvider:** Add tests for theme management, localStorage persistence, system preferences, and DOM updates.
- **Header & Footer:** Verify header rendering, user menu functionality, and footer content consistency.
- **AuthInitializer:** Ensure authentication state is correctly loaded from storage on mount.

frontend/src/components/dev/ComponentShowcase.tsx

@@ -1,6 +1,9 @@
+/* istanbul ignore file */
+
 /**
  * Component Showcase
  * Comprehensive display of all design system components
+ * This file is excluded from coverage as it's a demo/showcase page
  */
 
 'use client';