Frontend SDK: Enhance useLockups React Hook

[EneGab]

Summary

Added a useLockups hook to the React SDK that returns { data, isLoading, error, refetch } with 30-second caching, automatic retry on transient RPC failures, and render optimization backed by a new getLockupInfo method in the SDK client and 7 unit tests covering error, loading, and cache scenarios.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 📝 Documentation update
• 🔧 Smart contract improvement
• 🧪 Test coverage improvement
• 🏗️ CI/Build improvement

Related Issue

Closes #248

Changes Made

• Added LockupInfo interface and getLockupInfo(user) method to sdk/src/client.ts and exported from sdk/src/index.ts
• Added useLockups(user) hook to react/src/hooks.ts with 30s caching, 3-attempt exponential backoff retry, and a render guard that prevents unnecessary re-renders when data is unchanged
• Added Jest test setup (react/jest.config.js, test deps in react/package.json) and 7 unit tests in react/src/hooks.test.ts covering loading state, error exposure, retry logic, null lockup, cache bypass, and render count

Testing

How has this been tested?

• Rust unit tests pass (cargo test)
• SDK compiles (npm run build in sdk/)
• Manual testing against Soroban testnet
• New tests added for changes

Test commands run:

React hook tests

cd react && npx jest --no-coverage

Result: 7 passed, 7 total

## Checklist

- [x] My code follows the project's style guidelines
- [ ] I have added NatSpec-style comments to new Rust functions
- [x] I have added JSDoc comments to new TypeScript functions
- [ ] I have updated the README if needed
- [x] My branch follows the naming convention: `feature/<issue-number>-<description>`
- [x] I have not modified files outside the scope of this issue

## Drips.network Contributor Info

- **Drips Profile**: https://www.drips.network/wave/users/5004cb7e-4d58-4dcc-9250-a7acd5c53d11
- **Issue Claimed**: #248

## Screenshots (if applicable)

<!-- Add screenshots for UI changes or contract interaction examples -->

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[p3ris0n]

@EneGab please resolve conflicts