feat: Add React hooks wrapper package

[kevin-undefined]

Summary

This PR adds a React hooks wrapper package for the SoroSave SDK, making it easy to integrate SoroSave into React applications.

Changes

New Files

• src/react/context.tsx - SoroSaveProvider component and hooks for accessing client/config
• src/react/useGroup.ts - Hook for fetching and monitoring a savings group
• src/react/useContribute.ts - Mutation hook for making contributions
• src/react/useMemberGroups.ts - Hook for fetching all groups a member belongs to
• src/react/index.ts - Public exports for the React package

Modified Files

• package.json - Added React peer dependency and subpath exports for @sorosave/react
• tsconfig.json - Added JSX support for React components
• src/index.ts - Re-export React hooks
• README.md - Added comprehensive React hooks documentation

Usage

import { SoroSaveProvider, useGroup, useContribute, useMemberGroups } from '@sorosave/react';

function App() {
  return (
    <SoroSaveProvider config={{
      contractId: 'YOUR_CONTRACT_ID',
      rpcUrl: 'https://soroban-testnet.stellar.org',
      networkPassphrase: 'Test SDF Network ; September 2015',
    }}>
      <YourApp />
    </SoroSaveProvider>
  );
}

function GroupInfo({ groupId }) {
  const { group, loading, error } = useGroup(groupId);
  // ...
}

Acceptance Criteria

• Add useGroup(groupId) hook returning group data + loading + error
• Add useContribute() hook returning mutation function
• Add useMemberGroups(address) hook
• Add SoroSaveProvider context component
• Export as separate package via subpath exports

Closes #45