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