Add LittleShop.Client SDK library with complete API wrapper

Features:
- Complete .NET client SDK for LittleShop API
- JWT authentication with automatic token management
- Catalog service for products and categories
- Order service with payment creation
- Retry policies using Polly for resilience
- Error handling middleware
- Dependency injection support
- Comprehensive documentation and examples

SDK Components:
- Authentication service with token refresh
- Strongly-typed models for all API responses
- HTTP handlers for retry and error handling
- Extension methods for easy DI registration
- Example console application demonstrating usage

Test Updates:
- Fixed test compilation errors
- Updated test data builders for new models
- Corrected service constructor dependencies
- Fixed enum value changes (PaymentStatus, OrderStatus)

Documentation:
- Complete project README with features and usage
- Client SDK README with detailed examples
- API endpoint documentation
- Security considerations
- Deployment guidelines

Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

LittleShop.Client/LittleShopClient.cs

@@ -0,0 +1,29 @@
+using LittleShop.Client.Configuration;
+using LittleShop.Client.Services;
+using Microsoft.Extensions.Options;
+
+namespace LittleShop.Client;
+
+public interface ILittleShopClient
+{
+    IAuthenticationService Authentication { get; }
+    ICatalogService Catalog { get; }
+    IOrderService Orders { get; }
+}
+
+public class LittleShopClient : ILittleShopClient
+{
+    public IAuthenticationService Authentication { get; }
+    public ICatalogService Catalog { get; }
+    public IOrderService Orders { get; }
+    
+    public LittleShopClient(
+        IAuthenticationService authenticationService,
+        ICatalogService catalogService,
+        IOrderService orderService)
+    {
+        Authentication = authenticationService;
+        Catalog = catalogService;
+        Orders = orderService;
+    }
+}