Add E2E tests for internationalization across multiple pages

- Implemented navigation.spec.ts to test language switching and translation of navigation elements.
- Created sessions-page.spec.ts to verify translations on the sessions page, including headers, status badges, and date formatting.
- Developed settings-page.spec.ts to ensure settings page content is translated and persists across sessions.
- Added skills-page.spec.ts to validate translations for skill categories, action buttons, and empty states.

ccw/frontend/src/pages/NotFoundPage.tsx

@@ -0,0 +1,52 @@
+// ========================================
+// 404 Not Found Page
+// ========================================
+// Displayed when user navigates to a non-existent route
+
+import { Link } from 'react-router-dom';
+import { Home, ArrowLeft } from 'lucide-react';
+import { Button } from '@/components/ui/Button';
+import { Card } from '@/components/ui/Card';
+
+export function NotFoundPage() {
+  return (
+    <div className="min-h-[400px] flex items-center justify-center">
+      <Card className="max-w-md w-full p-8 text-center">
+        {/* 404 Number */}
+        <div className="text-6xl font-bold text-primary mb-4">404</div>
+
+        {/* Message */}
+        <h1 className="text-2xl font-bold text-foreground mb-2">
+          Page Not Found
+        </h1>
+        <p className="text-muted-foreground mb-6">
+          The page you're looking for doesn't exist or has been moved.
+        </p>
+
+        {/* Action Buttons */}
+        <div className="flex flex-col sm:flex-row gap-3 justify-center">
+          <Button variant="default" asChild>
+            <Link to="/">
+              <Home className="w-4 h-4 mr-2" />
+              Go Home
+            </Link>
+          </Button>
+          <Button variant="outline" onClick={() => window.history.back()}>
+            <ArrowLeft className="w-4 h-4 mr-2" />
+            Go Back
+          </Button>
+        </div>
+
+        {/* Help Link */}
+        <p className="text-sm text-muted-foreground mt-6">
+          Need help? Visit the{' '}
+          <Link to="/help" className="text-primary hover:underline">
+            Help page
+          </Link>
+        </p>
+      </Card>
+    </div>
+  );
+}
+
+export default NotFoundPage;