FHEVM SDK refactoring with complete React integration.

[gregory-yang243u]

FEATURES:

• Core SDK Implementation: FhevmClient, error handling, and preset configurations
• React Integration: useFhevmEncrypt, useFhevmDecrypt, useFhevmInstance hooks
• Comprehensive Testing: 425+ lines of unit tests covering all core functionality
• Demo Component: SimplifiedFHECounterDemo showcasing practical usage
• Configuration Updates: Updated package.json and module exports

MODULES:

1. Core SDK

   • FhevmClient.ts: Main client implementation
   • createFhevmClient.ts: Factory function
   • errors.ts: Error handling
   • presets.ts: Configuration presets

2. React Hooks

   • useFhevmEncrypt.ts: Encryption hook
   • useFhevmDecrypt.ts: Decryption hook
   • useFhevmInstance.ts: Instance management
   • useFhevm.tsx: Enhanced main hook

3. Tests

   • FhevmClient.test.ts: Client tests
   • errors.test.ts: Error handling tests
   • presets.test.ts: Preset tests

4. Demo

   • SimplifiedFHECounterDemo.tsx: