Update react frontend engineer chat mode

[kristiyan-velkov]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• My contribution adds a new instruction, prompt, or chat mode file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, or chat mode with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.

---

Description

This PR updates comprehensive React.js Front-End Specialist. Added chat mode designed for developers building production-ready applications using React 19 and modern front-end best practices.

---

Type of Contribution

• Update to existing instruction, prompt, chat mode, or collection.

---

Key Features:

• React 19.2 Support with View Transitions, useEffectEvent(), and <Activity/> component
• React Server Components (RSC) for improved performance and data-fetching efficiency
• Automatic Batching & Transitions for smoother UI updates and reduced re-renders
• React Compiler Integration enabling automatic memoization without manual optimization
• Concurrent Rendering for non-blocking updates and better responsiveness
• TypeScript Integration with strict typing for props, hooks, and component patterns
• Advanced State Management with Context API, Redux Toolkit, and Zustand
• Performance Optimization using lazy loading, code splitting, and Suspense boundaries
• Routing Patterns using React Router 7.4 with nested routes and data APIs
• Error Handling through error boundaries and async component recovery
• Accessibility & Progressive Enhancement following WCAG and modern UX best practices
• Testing Integration with Jest, React Testing Library, and Playwright for E2E testing
• Deployment Ready for Vercel, Netlify, Docker, or self-hosted environments

This chat mode is particularly useful for teams building modern React.js 19 applications that demand performance, scalability, type safety, and design system consistency while adhering to the latest React Server Components and React Compiler architecture.

By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.