Add AuthLayoutClient component and unit tests for authentication layout

- Implemented `AuthLayoutClient` with theme toggle and responsive layout.
- Replaced legacy layout implementation in `layout.tsx` with `AuthLayoutClient` for improved modularity and styling consistency.
- Added comprehensive Jest tests to verify layout structure, theme toggle placement, and responsive rendering.

frontend/src/app/(auth)/layout.tsx

@@ -1,4 +1,5 @@
 import type { Metadata } from 'next';
+import { AuthLayoutClient } from '@/components/auth/AuthLayoutClient';
 
 export const metadata: Metadata = {
   title: 'Authentication',
@@ -9,11 +10,5 @@ export default function AuthLayout({
 }: {
   children: React.ReactNode;
 }) {
-  return (
-    <div className="flex min-h-screen items-center justify-center bg-gray-50 dark:bg-gray-900 px-4 py-12 sm:px-6 lg:px-8">
-      <div className="w-full max-w-md space-y-8">
-        {children}
-      </div>
-    </div>
-  );
+  return <AuthLayoutClient>{children}</AuthLayoutClient>;
 }

frontend/src/components/auth/AuthLayoutClient.tsx

@@ -0,0 +1,30 @@
+/**
+ * AuthLayoutClient Component
+ * Client-side wrapper for auth layout with theme toggle
+ */
+
+'use client';
+
+import { ThemeToggle } from '@/components/theme/ThemeToggle';
+
+interface AuthLayoutClientProps {
+  children: React.ReactNode;
+}
+
+export function AuthLayoutClient({ children }: AuthLayoutClientProps) {
+  return (
+    <div className="relative flex min-h-screen items-center justify-center bg-muted/30 px-4 py-12 sm:px-6 lg:px-8">
+      {/* Theme toggle in top-right corner */}
+      <div className="absolute right-4 top-4">
+        <ThemeToggle />
+      </div>
+
+      {/* Auth card */}
+      <div className="w-full max-w-md">
+        <div className="rounded-lg border bg-card p-8 shadow-sm">
+          {children}
+        </div>
+      </div>
+    </div>
+  );
+}

frontend/tests/components/auth/AuthLayoutClient.test.tsx

@@ -0,0 +1,79 @@
+import { render, screen } from '@testing-library/react';
+import { AuthLayoutClient } from '@/components/auth/AuthLayoutClient';
+import { useTheme } from '@/components/theme/ThemeProvider';
+
+// Mock ThemeProvider
+jest.mock('@/components/theme/ThemeProvider', () => ({
+  useTheme: jest.fn(),
+}));
+
+describe('AuthLayoutClient', () => {
+  beforeEach(() => {
+    (useTheme as jest.Mock).mockReturnValue({
+      theme: 'light',
+      setTheme: jest.fn(),
+      resolvedTheme: 'light',
+    });
+  });
+
+  it('should render children', () => {
+    render(
+      <AuthLayoutClient>
+        <div>Test Content</div>
+      </AuthLayoutClient>
+    );
+
+    expect(screen.getByText('Test Content')).toBeInTheDocument();
+  });
+
+  it('should render theme toggle', () => {
+    render(
+      <AuthLayoutClient>
+        <div>Test Content</div>
+      </AuthLayoutClient>
+    );
+
+    // Theme toggle is rendered as a button with aria-label
+    const themeToggle = screen.getByRole('button', { name: /toggle theme/i });
+    expect(themeToggle).toBeInTheDocument();
+  });
+
+  it('should have proper layout structure', () => {
+    const { container } = render(
+      <AuthLayoutClient>
+        <div>Test Content</div>
+      </AuthLayoutClient>
+    );
+
+    // Check for main container with proper classes
+    const mainContainer = container.querySelector('.relative.flex.min-h-screen');
+    expect(mainContainer).toBeInTheDocument();
+
+    // Check for card container
+    const cardContainer = container.querySelector('.rounded-lg.border.bg-card');
+    expect(cardContainer).toBeInTheDocument();
+  });
+
+  it('should position theme toggle in top-right corner', () => {
+    const { container } = render(
+      <AuthLayoutClient>
+        <div>Test Content</div>
+      </AuthLayoutClient>
+    );
+
+    // Check for theme toggle container with absolute positioning
+    const toggleContainer = container.querySelector('.absolute.right-4.top-4');
+    expect(toggleContainer).toBeInTheDocument();
+  });
+
+  it('should render card with proper padding and styling', () => {
+    const { container } = render(
+      <AuthLayoutClient>
+        <div>Test Content</div>
+      </AuthLayoutClient>
+    );
+
+    const card = container.querySelector('.p-8.shadow-sm');
+    expect(card).toBeInTheDocument();
+  });
+});