Add unit tests for React components with Testing Library #76
Description
Set up React Testing Library and write tests for core components.
Acceptance Criteria:
- Install and configure @testing-library/react with Jest
- Write tests for: GroupCard, ConnectWallet, ContributeModal, Navbar
- Test user interactions (click, input, form submit)
- Add test script to package.json and CI
Files: new files src/components/__tests__/*.test.tsx, package.json, jest.config.js