test: implement OrderItems display component tests (#15)

[LudoLoops]

Summary

Implemented comprehensive tests for OrderItems display component (task #15) following TDD methodology.

• Created 30+ test cases covering table rendering, item display, and calculations
• Tests include empty state handling, mobile responsiveness, and accessibility
• All tests follow Vitest browser testing pattern with Playwright
• Tests verify product display, margin calculations, and interactive buttons

Test Coverage

• Table Rendering: Display all items with product names, quantities, costs, revenues, margins, and percentages
• Item Numbering: Verify items are numbered starting from 1
• Buttons: Test edit and delete buttons are accessible and properly labeled
• Totals: Test order totals section displays cost, revenue, margin, and profit percentage
• Empty State: Test proper message when no items in order
• Mobile: Test responsive layout for mobile devices
• Accessibility: Test semantic HTML structure and ARIA labels

Implementation Status

• ✅ Tests written (RED phase - tests fail as component exists but not fully implemented)
• ✅ Component created with basic table and totals display
• ✅ Roadmap updated: checked off 1.5a (ProductGrid component) and 1.6 (OrderItems tests)

Next Steps

1. Run full test suite to ensure tests pass
2. Proceed to 1.6a - Implement OrderItems component (GREEN phase)
3. Then refactor for clean code (REFACTOR phase)
4. Continue to 1.7 - Main Home Page / Layout

Closes #15