test: add frontend test setup with Vitest

[Abhishek-YS]

Summary

• Added Vitest with jsdom configuration
• Added React Testing Library, jest-dom, and user-event
• Added frontend test script
• Added starter utility tests for healthUtils
• Added starter component test for FitnessChatBot
• Added browser API mocks for the test environment
• Updated GitHub Actions workflow to run frontend tests in CI

Testing

npm test -- --run