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>;
 }