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/tests/components/auth/AuthInitializer.test.tsx

@@ -0,0 +1,58 @@
+/**
+ * Tests for AuthInitializer
+ * Verifies authentication state is loaded from storage on mount
+ */
+
+import { render, waitFor } from '@testing-library/react';
+import { AuthInitializer } from '@/components/auth/AuthInitializer';
+import { useAuthStore } from '@/stores/authStore';
+
+// Mock the auth store
+jest.mock('@/stores/authStore', () => ({
+  useAuthStore: jest.fn(),
+}));
+
+describe('AuthInitializer', () => {
+  const mockLoadAuthFromStorage = jest.fn();
+
+  beforeEach(() => {
+    jest.clearAllMocks();
+
+    (useAuthStore as unknown as jest.Mock).mockImplementation((selector: any) => {
+      const state = {
+        loadAuthFromStorage: mockLoadAuthFromStorage,
+      };
+      return selector(state);
+    });
+  });
+
+  describe('Initialization', () => {
+    it('renders nothing (null)', () => {
+      const { container } = render(<AuthInitializer />);
+
+      expect(container.firstChild).toBeNull();
+    });
+
+    it('calls loadAuthFromStorage on mount', async () => {
+      render(<AuthInitializer />);
+
+      await waitFor(() => {
+        expect(mockLoadAuthFromStorage).toHaveBeenCalledTimes(1);
+      });
+    });
+
+    it('does not call loadAuthFromStorage again on re-render', async () => {
+      const { rerender } = render(<AuthInitializer />);
+
+      await waitFor(() => {
+        expect(mockLoadAuthFromStorage).toHaveBeenCalledTimes(1);
+      });
+
+      // Force re-render
+      rerender(<AuthInitializer />);
+
+      // Should still only be called once (useEffect dependencies prevent re-call)
+      expect(mockLoadAuthFromStorage).toHaveBeenCalledTimes(1);
+    });
+  });
+});