Feature: Add product variant price override support

Enables individual variants to have their own prices, overriding the base product price.

**Database Changes:**
- Added Price (decimal?, nullable) to ProductVariants table
- Added ProductVariantId to OrderItems table with foreign key relationship
- Created index on OrderItems.ProductVariantId for performance

**API Changes:**
- ProductVariantDto: Added Price field
- CreateProductVariantDto: Added Price field with validation
- UpdateProductVariantDto: Added Price field
- OrderItemDto: Added ProductVariantId and ProductVariantName
- CreateOrderItemDto: Added ProductVariantId

**Business Logic:**
- OrderService: Variant price overrides base price (but multi-buy takes precedence)
- ProductService: All variant CRUD operations support Price field

**Admin UI:**
- CreateVariant: Price input with £ symbol and base price placeholder
- EditVariant: Price editing with £ symbol
- ProductVariants list: Shows variant price or "(base)" indicator

**Client Library:**
- Updated all DTOs to match server-side changes
- Full support for variant pricing in order creation

**Migration:**
- EF Core migration: 20251003173458_AddVariantPricing
- Backward compatible: NULL values supported for existing data

**Use Case:**
Products with size/color variants can now have different prices:
- Small T-shirt: £15.00 (variant override)
- Medium T-shirt: £18.00 (uses base price)
- Large T-shirt: £20.00 (variant override)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

LittleShop/DTOs/ProductDto.cs

@@ -125,6 +125,7 @@ public class ProductVariantDto
     public string VariantType { get; set; } = "Standard";
     public int SortOrder { get; set; }
     public int StockLevel { get; set; }
+    public decimal? Price { get; set; } // If null, uses product.Price
     public bool IsActive { get; set; }
     public DateTime CreatedAt { get; set; }
     public DateTime UpdatedAt { get; set; }
@@ -170,6 +171,9 @@ public class CreateProductVariantDto
 
     [Range(0, int.MaxValue)]
     public int StockLevel { get; set; } = 0;
+
+    [Range(0.01, double.MaxValue, ErrorMessage = "Price must be greater than 0")]
+    public decimal? Price { get; set; } // Optional: If null, uses product.Price
 }
 
 public class UpdateProductMultiBuyDto
@@ -205,5 +209,8 @@ public class UpdateProductVariantDto
     [Range(0, int.MaxValue)]
     public int? StockLevel { get; set; }
 
+    [Range(0.01, double.MaxValue, ErrorMessage = "Price must be greater than 0")]
+    public decimal? Price { get; set; } // Optional: If null, uses product.Price
+
     public bool? IsActive { get; set; }
 }