Add Cart functionality and enhanced Copilot instructions

[yortch]

Summary

This PR adds shopping cart functionality to the OctoCAT Supply application and includes comprehensive GitHub Copilot instructions for the project.

Changes

Cart Functionality

• Added CartContext.tsx: Implements shopping cart state management with add, remove, update quantity, and clear operations
• Added Cart.tsx component: Full cart UI with item display, quantity controls, and checkout flow
• Updated Navigation.tsx: Added cart icon with item count badge in the navigation bar
• Updated Products.tsx: Integrated "Add to Cart" functionality for products

Documentation

• Added .github/copilot-instructions.md: Comprehensive project documentation including:
  • Project overview and architecture
  • Data model and ERD structure
  • Development patterns and conventions
  • In-memory storage implementation details
  • Testing guidelines
  • Demo-specific features and workshop scenarios

Files Changed

• .github/copilot-instructions.md (new)
• frontend/src/context/CartContext.tsx (new)
• frontend/src/components/Cart.tsx (new)
• frontend/src/components/Navigation.tsx (modified)
• frontend/src/components/entity/product/Products.tsx (modified)
• frontend/src/App.tsx (modified)

Testing

• Cart context provides state management following the existing context pattern (AuthContext, ThemeContext)
• UI components follow Tailwind CSS styling conventions used throughout the project

Notes

This enhances the demo application with a complete shopping cart experience, making it more suitable for showcasing GitHub Copilot capabilities in a realistic e-commerce scenario.