feat: add initial implementation of Kitchen CRM with authentication and dashboard features

- Create global styles and theme management
- Implement app shell layout with sidebar navigation
- Add authentication layout and pages for login and registration
- Develop dashboard page with placeholder content
- Introduce routing guards for guest-only and authenticated routes
- Set up Zustand for state management of authentication and theme
- Create API types and structures for CRM entities
- Configure Vite with PWA support and Tailwind CSS

frontend/src/pages/not-found-page.tsx

@@ -0,0 +1,14 @@
+import { Link } from 'react-router-dom'
+
+export const NotFoundPage = () => (
+  <div className="flex min-h-[60vh] flex-col items-center justify-center space-y-4 text-center">
+    <div className="space-y-2">
+      <p className="text-sm font-semibold text-primary">404</p>
+      <h1 className="text-3xl font-bold tracking-tight text-foreground">Страница не найдена</h1>
+      <p className="text-sm text-muted-foreground">Мы не смогли найти запрошенную страницу. Проверьте URL или вернитесь на дашборд.</p>
+    </div>
+    <Link className="text-sm font-semibold text-primary hover:underline" to="/dashboard">
+      Вернуться в приложение
+    </Link>
+  </div>
+)