add frontend files

app/javascript/src/index.tsx

@@ -0,0 +1,12 @@
+import React from 'react'
+import ReactDOM from 'react-dom'
+import { App } from './App'
+
+document.addEventListener('DOMContentLoaded', () => {
+  ReactDOM.render(
+    <React.StrictMode>
+      <App />
+    </React.StrictMode>,
+    document.getElementById('root'),
+  )
+})