Implement comprehensive UI tests for all CSS grid features with Playwright and CI/CD integration

[Copilot]

This PR implements a complete UI testing suite for all CSS grid features provided by the grid-bootstrap5 library, addressing the need for comprehensive visual testing and regression prevention.

What's Included

🎯 Comprehensive Test Coverage

• Complete feature testing for all grid utilities: d-grid, grid-cols-*, grid-cs-*, grid-rs-*, grid-cg-*, grid-rg-*, and just-self-*
• Responsive testing across all Bootstrap breakpoints (XS, SM, MD, LG, XL, XXL)
• Visual regression testing with screenshot comparison capabilities
• Cross-browser support with Chromium and mobile browser testing

📱 Test Pages Created

• Comprehensive test page (tests/ui/pages/comprehensive-test.html) - Tests all 50+ grid classes in organized sections
• Responsive test page (tests/ui/pages/responsive-test.html) - Validates mobile-first responsive behavior
• Test index page (tests/ui/index.html) - Navigation hub with feature documentation

🔬 Modern Testing Framework

• Playwright integration for reliable UI testing with screenshot capabilities
• Automated test scripts that validate all grid features programmatically
• Multiple viewport testing (mobile: 375px, tablet: 768px, desktop: 1200px+)
• Visual artifacts captured and stored for regression analysis

🚀 CI/CD Integration

• GitHub Actions workflow updated to run UI tests on all pull requests
• Automated screenshot capture with artifacts uploaded for visual review
• Non-blocking testing that doesn't interfere with existing build processes
• Standalone test runner that works without rebuilding CSS dependencies

📊 Quality Assurance Features

The test suite validates:

• Grid layout rendering across different column counts (1-12 columns)
• Column and row spanning behavior with various span values
• Grid gap utilities with different spacing values (0-5)
• Responsive grid behavior at different breakpoints
• Complex real-world layout scenarios (headers, sidebars, content areas)
• Justify-self alignment utilities (start, end, center, stretch)

🛠️ Developer Experience

• Easy local testing with npm test or npm run serve
• Interactive test UI with npm run test:ui
• Comprehensive documentation in tests/ui/README.md
• Baseline screenshots included for immediate visual comparison

Example Usage

# Run all tests
npm test

# View tests manually in browser
npm run serve
# Then open: http://localhost:3000/tests/ui/

# Run tests with interactive UI
npm run test:ui

The implementation ensures that all CSS grid mixins and utilities work correctly across devices and browsers, providing confidence for future development and preventing visual regressions.

Fixes #1.

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