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>
+  );
+}