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/Services/ProductService.cs

@@ -556,6 +556,7 @@ public class ProductService : IProductService
             VariantType = createVariantDto.VariantType,
             SortOrder = createVariantDto.SortOrder,
             StockLevel = createVariantDto.StockLevel,
+            Price = createVariantDto.Price,
             IsActive = true,
             CreatedAt = DateTime.UtcNow,
             UpdatedAt = DateTime.UtcNow
@@ -572,6 +573,7 @@ public class ProductService : IProductService
             VariantType = variant.VariantType,
             SortOrder = variant.SortOrder,
             StockLevel = variant.StockLevel,
+            Price = variant.Price,
             IsActive = variant.IsActive,
             CreatedAt = variant.CreatedAt,
             UpdatedAt = variant.UpdatedAt
@@ -595,6 +597,9 @@ public class ProductService : IProductService
         if (updateVariantDto.StockLevel.HasValue)
             variant.StockLevel = updateVariantDto.StockLevel.Value;
 
+        if (updateVariantDto.Price.HasValue)
+            variant.Price = updateVariantDto.Price.Value;
+
         if (updateVariantDto.IsActive.HasValue)
             variant.IsActive = updateVariantDto.IsActive.Value;
 
@@ -627,6 +632,7 @@ public class ProductService : IProductService
                 VariantType = v.VariantType,
                 SortOrder = v.SortOrder,
                 StockLevel = v.StockLevel,
+                Price = v.Price,
                 IsActive = v.IsActive,
                 CreatedAt = v.CreatedAt,
                 UpdatedAt = v.UpdatedAt
@@ -647,6 +653,7 @@ public class ProductService : IProductService
             VariantType = variant.VariantType,
             SortOrder = variant.SortOrder,
             StockLevel = variant.StockLevel,
+            Price = variant.Price,
             IsActive = variant.IsActive,
             CreatedAt = variant.CreatedAt,
             UpdatedAt = variant.UpdatedAt