fix: replace deprecated ReactDOM.render with createRoot API

[AliMahmoudDev]

What this PR does

Replaces the deprecated ReactDOM.render with the modern createRoot API from React 18+.

Changes

• Replaced ReactDOM.render() with createRoot().render()
• Added null check for root container with helpful error message
• Wrapped in React.StrictMode for development warnings
• This fixes the deprecation warning shown in the console

Closes #16

Before

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

After

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