Migrate render-component.tsx to React 18 createRoot API

## Description

Migrate `packages/react/app/src/render-component.tsx` from the deprecated `ReactDOM.render()` API to the React 18 `createRoot()` API.

## Current State
- Currently uses `ReactDOM.render()` and `ReactDOM.unmountComponentAtNode()` (deprecated in React 18)
- Already has commented React 18 implementation using `createRoot()`

## Implementation Steps
1. Replace `ReactDOM.render()` with `createRoot().render()`
2. Replace `ReactDOM.unmountComponentAtNode()` with `root.unmount()`
3. Remove the deprecated `renderComponent` and `renderApp` functions
4. Update imports to use `createRoot` from 'react-dom/client'

## Files to Update
- `packages/react/app/src/render-component.tsx`
- `packages/react/app/src/render-app.ts` (remove deprecated function)

## Testing
- Ensure existing functionality still works
- Test both mounting and unmounting scenarios
- Verify compatibility with existing cookbooks

## Labels
👾 React, 🧨 breaking changes

Start with packages/react/app/src/render-component.tsx and compare the current implementation with its commented React 18 version; then inspect packages/react/app/src/render-app.ts for the deprecated function. The work is done when mounting and unmounting still function through the React 18 API and existing cookbooks remain compatible.

Written by the indexing model from the issue text.