Add comprehensive E-commerce sample application with Azure Cosmos DB and GitHub Copilot integration

[Copilot]

This PR introduces a complete E-commerce sample application that demonstrates modern web development best practices using Azure Cosmos DB, along with specialized GitHub Copilot customizations for enhanced development productivity.

🛍️ Complete E-commerce Application

The sample includes a full-stack application with:

• Backend: Node.js + Express API with comprehensive Azure Cosmos DB integration
• Frontend: React + TypeScript with responsive design and modern UI components
• Database: Optimized container design demonstrating real-world partition strategies
• DevOps: Docker configuration and deployment-ready setup

🎯 Key Features Demonstrated

Azure Cosmos DB Best Practices

• Optimal Partition Strategies: Products by category, Orders/Cart by user ID for efficient querying
• Query Optimization: Point reads, composite indexes, and parameterized queries
• Data Modeling: Schema-free design with proper document structure for e-commerce workloads
• Performance Patterns: Request Unit optimization, connection pooling, and caching strategies

E-commerce Functionality

• Product Catalog: Advanced search, filtering, and category browsing
• Shopping Cart: Real-time cart management with inventory validation
• Order Processing: Complete workflow from cart to order tracking
• User Management: Profile management and address handling

🤖 GitHub Copilot Integration

Custom Instructions

• instructions/ecommerce-cosmosdb.instructions.md: Comprehensive guidance for e-commerce development with Azure Cosmos DB
• instructions/ecommerce-development.chatmode.md: Specialized chat mode for e-commerce scenarios

Reusable Prompts

• Product catalog management workflows
• Shopping cart and session handling
• Order processing and fulfillment patterns

🚀 Production-Ready Features

• Error Handling: Cosmos DB specific error codes with proper retry logic
• Security: Input validation, parameterized queries, and secure environment management
• Scalability: Container design optimized for global distribution and high throughput
• Documentation: Comprehensive setup guides and data model documentation

📁 Structure

samples/ecommerce-app/
├── src/
│   ├── backend/          # Node.js API server
│   └── frontend/         # React TypeScript app
├── docs/                 # Comprehensive documentation
├── docker-compose.yml    # Local development setup
└── validate.sh          # Application validation script

The sample serves as both a learning resource and a foundation for building production e-commerce applications with Azure Cosmos DB, while showcasing how GitHub Copilot can accelerate development with proper customizations.

Fixes #6.

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