Add App.js component to client/src for React application structure

- Created App.js in client/src as the main application component for the React app.
- Defines a basic functional component that renders a "Hello, World!" message.
- Supports the main entry point (index.js) by providing a component to render to the DOM.

This addition completes the basic structure required for the React application to build successfully.

Author: KazerL

client/src/App.js

@@ -0,0 +1,11 @@
+import React from 'react';
+
+function App() {
+  return (
+    <div className="App">
+      <h1>Hello, World!</h1>
+    </div>
+  );
+}
+
+export default App;