feat: add React renderer and sample clients

[abalmush]

• Add @a2ui/react renderer with ShadCN-inspired Tailwind components
• Implement core A2UI components: Text, Button, Card, Row, Column, List, etc.
• Add A2UIProvider and ThemeProvider context providers
• Add useA2UI and useDataBinding hooks for state management
• Include 64 unit tests with full coverage
• Add React sample clients: contact, restaurant, gallery, rizzcharts, orchestrator
• Add a2a-chat-canvas component for chat interfaces

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[MarianaDn]

Great contribution to the project.
Having a React renderer with well-designed core components and examples significantly improves developer experience and discoverability.
This is a very useful foundation for future extensions.

[hkulas]

Looks great, would help our project as well!

[lukasmoschitz]

Hi @abalmush,
Thank you for this contribution it looks great!
We appreciate the effort you put into the React renderer, components, and test coverage.
We wanted to share that we're collaborating with Google and the React Working Group on the official React renderer for A2UI. Your work here, especially the component patterns and data binding approach, has been helpful as we've explored the design space.
We'd love to have you involved in the ongoing effort: #542
Thanks for helping push A2UI forward!