feat: add React client demo for A2UI rendering

[PranavKohli24]

The Roadmap mentions adding official support for React. I have initialized the samples/client/react directory and added a basic boilerplate using Vite.

It includes a simple proof-of-concept renderer that maps A2UI JSON (text and buttons) to React components. This serves as a foundation for future React implementation.

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[PranavKohli24]

I have signed the CLA.

[PranavKohli24]

I linked my account.

[zeroasterisk]

Thanks for getting the React renderer conversation started with this POC!

We now have an official React renderer in progress at #542, led by @lukasmoschitz (CopilotKit + Google collaboration). The spec is also moving from v0.8 to v0.9 soon, with shared libraries across renderers.

If you're interested in continuing to contribute, we'd welcome your help reviewing and contributing to #542. Closing this in favor of the consolidated effort — thanks again for the initiative!