add back App.tsx

Author: moribellamy

internal/dev_server/ui/src/App.tsx

@@ -0,0 +1,10 @@
+import './App.css';
+import FlagsPage from './FlagsPage.tsx';
+
+function App() {
+  return (
+    <FlagsPage />
+  )
+}
+
+export default App;