feat(frontend): add specialized readme.md for React

[beginwebdev2002]

This PR adds a comprehensive suite of modern React (v19+) documentation inside the frontend/ directory, adhering strictly to the "2026 frontend standard" vibe-coding patterns requested.

Changes:

1. React Directory Creation: Created frontend/react/ with an initial readme.md acting as the technology entry point. Features explicit AI constraints and the required YAML SEO frontmatter.
2. Modularization: Extracted dense concepts into specialized sub-files:
   • performance.md: Discusses the React Compiler and use() API.
   • state-management.md: Covers Server Actions and the useActionState hook.
3. Cross-linking: Added bidirectional linking to React within the root frontend/readme.md index.
4. No Extraneous Edits: Ensure all work remains solely inside the frontend/ directory to satisfy strict constraints.

---

PR created automatically by Jules for task 10326564130571187815 started by @beginwebdev2002

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.