Modernize React code

Prepare a file for React 19. In version 19 the following were removed:

• unmountComponentAtNode
• findDOMNode
• ReactDOM.render
• Legacy Context using contextTypes and getChildContext
• propTypes and defaultProps for functions

Also:

• useRef requires an argument

Migrating to createRoot

Replace all instances of ReactDOM.render with the new createRoot API.

Before

import ReactDOM from 'react-dom';

ReactDOM.render(<App />, document.getElementById('root'));

After

import { createRoot } from 'react-dom/client';

const root = createRoot(document.getElementById('root'));
root.render(<App />);

Notes

• The root element must exist in the DOM before calling createRoot
• To unmount, use root.unmount() instead of ReactDOM.unmountComponentAtNode
• Error boundaries work the same way with createRoot
• Hydration is done using hydrateRoot instead of ReactDOM.hydrate
• When migrating from ReactDOM.render to createRoot, consider if flushSync is needed for old patterns.