feat(frontend): add minimal React source files for Docker build

- Add App.tsx with placeholder content
- Add main.tsx entry point
- Add index.css with basic styles

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

german-app-frontend/src/App.tsx

@@ -0,0 +1,8 @@
+export default function App() {
+  return (
+    <div>
+      <h1>DeutschLernen</h1>
+      <p>German Learning Application</p>
+    </div>
+  );
+}

german-app-frontend/src/index.css

@@ -0,0 +1,9 @@
+* {
+  margin: 0;
+  padding: 0;
+  box-sizing: border-box;
+}
+
+body {
+  font-family: Arial, sans-serif;
+}

german-app-frontend/src/main.tsx

@@ -0,0 +1,5 @@
+import { createRoot } from 'react-dom/client';
+import App from './App';
+import './index.css';
+
+createRoot(document.getElementById('root')!).render(<App />);