feat: implement NotFoundPage (404) and fix Cypress config

- Implement NotFoundPage with required test IDs for 404 error page
- Add data-testids: error-404-title, error-message, back-button, home-link, error-heading
- Fix Cypress baseUrl config from 3003/components to localhost:3001
- Clean up .js files in pages directory (keep only .tsx)
- App passes Task 1.1 requirements (404 page structure complete)

apps/react/src/app/pages/NotFoundPage.tsx

@@ -1,11 +1,35 @@
 import React from 'react'
+import { useNavigate } from 'react-router-dom'
 
 export const NotFoundPage: React.FC = () => {
+  const navigate = useNavigate()
+
   return (
-    <div className="page-not-found">
-      <h1>404 - Page Not Found</h1>
-      <p>The page you are looking for does not exist.</p>
-      <a href="/">Back to Home</a>
+    <div className="page-not-found" data-testid="page-not-found">
+      <div className="page-not-found__content">
+        <h1 className="page-not-found__title" data-testid="error-heading" role="heading">
+          <span data-testid="error-404-title">404</span>
+        </h1>
+        <p className="page-not-found__message" data-testid="error-message">
+          The page you are looking for does not exist.
+        </p>
+        <div className="page-not-found__actions">
+          <button
+            className="page-not-found__back-button"
+            data-testid="back-button"
+            onClick={() => navigate(-1)}
+          >
+            Go Back
+          </button>
+          <a
+            href="/"
+            className="page-not-found__home-link"
+            data-testid="home-link"
+          >
+            Go to Home
+          </a>
+        </div>
+      </div>
     </div>
   )
 }

e2e/cypress.config.ts

@@ -2,7 +2,7 @@ import { defineConfig } from 'cypress'
 
 export default defineConfig({
   e2e: {
-    baseUrl: 'http://127.0.0.1:3003/components',
+    baseUrl: 'http://localhost:3001',
     supportFile: 'cypress/support/index.ts',
     specPattern: ['cypress/integration/**/*.cy.ts', 'cypress/integration/**/*.spec.ts'],
     viewportWidth: 1440,