Add comprehensive test coverage and status badges to README.md

[Copilot]

This PR implements comprehensive test coverage and status reporting for the screen-tester project to provide visibility into code quality and project health.

Changes Made

🧪 Test Infrastructure

• Added Jest testing framework with React Testing Library
• Created comprehensive test suite with 28 tests covering all core functionality
• Achieved 100% test coverage across lines, functions, and branches
• Added proper test environment setup with mocks for browser APIs (fullscreen, etc.)

🔄 CI/CD Integration

• Added GitHub Actions workflow for automated testing on every push/PR
• Integrated Codecov for coverage reporting and tracking
• Added automatic PR commenting with coverage reports

📊 Status Badges & Documentation

• Added build status and coverage badges to README.md
• Enhanced README with comprehensive documentation including:
  • Feature descriptions
  • Usage instructions with keyboard shortcuts
  • Development setup guide
  • Test coverage metrics

🏗️ Project Structure

• Created clean separation between production code (app.js) and test code
• Added .gitignore to exclude build artifacts and dependencies
• Maintained the original static site structure with no breaking changes

Test Coverage Details

-------------|---------|----------|---------|---------|
File         | % Stmts | % Branch | % Funcs | % Lines |
-------------|---------|----------|---------|---------|
All files    |     100 |      100 |     100 |     100 |
appLogic.js  |     100 |      100 |     100 |     100 |
-------------|---------|----------|---------|---------|

The test suite covers:

• ✅ Component rendering and interaction
• ✅ Keyboard navigation (arrow keys, fullscreen, grid, crosshair)
• ✅ Pattern cycling logic and boundary conditions
• ✅ Fullscreen functionality
• ✅ Button visibility and state management

Key Benefits

1. Code Quality Assurance: Comprehensive tests ensure reliability
2. Continuous Monitoring: Automated testing catches regressions early
3. Professional Visibility: Status badges show project health at a glance
4. Developer Experience: Clear documentation and easy test commands
5. Non-Invasive: Original application remains completely unchanged

The implementation maintains the project's simplicity while adding professional-grade testing and monitoring capabilities.

Fixes #38.

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