feat: Initialize frontend with React, Vite, and Tailwind CSS

- Set up main entry point for React application.
- Create About, Home, NotFound, Privacy, and Terms pages with SEO support.
- Implement API service for file uploads and task management.
- Add global styles using Tailwind CSS.
- Create utility functions for SEO and text processing.
- Configure Vite for development and production builds.
- Set up Nginx configuration for serving frontend and backend.
- Add scripts for cleanup of expired files and sitemap generation.
- Implement deployment script for production environment.

frontend/src/pages/NotFoundPage.tsx

@@ -0,0 +1,34 @@
+import { Link } from 'react-router-dom';
+import { useTranslation } from 'react-i18next';
+import { Helmet } from 'react-helmet-async';
+import { Home } from 'lucide-react';
+
+export default function NotFoundPage() {
+  const { t } = useTranslation();
+
+  return (
+    <>
+      <Helmet>
+        <title>404 — {t('common.appName')}</title>
+        <meta name="robots" content="noindex" />
+      </Helmet>
+
+      <div className="flex flex-col items-center justify-center py-24 text-center">
+        <p className="text-7xl font-bold text-primary-600">404</p>
+        <h1 className="mt-4 text-2xl font-semibold text-slate-900">
+          Page Not Found
+        </h1>
+        <p className="mt-2 text-slate-500">
+          The page you're looking for doesn't exist or has been moved.
+        </p>
+        <Link
+          to="/"
+          className="btn-primary mt-8 inline-flex items-center gap-2"
+        >
+          <Home className="h-4 w-4" />
+          {t('common.home')}
+        </Link>
+      </div>
+    </>
+  );
+}