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/Models/OrderItem.cs

@@ -14,6 +14,8 @@ public class OrderItem
 
     public Guid? ProductMultiBuyId { get; set; } // Nullable for backward compatibility
 
+    public Guid? ProductVariantId { get; set; } // Optional: specific variant with potential price override
+
     [StringLength(500)]
     public string? SelectedVariants { get; set; } // JSON array of variants chosen (e.g., ["Red", "Blue", "Green"] for multi-buy)
 
@@ -29,4 +31,5 @@ public class OrderItem
     public virtual Order Order { get; set; } = null!;
     public virtual Product Product { get; set; } = null!;
     public virtual ProductMultiBuy? ProductMultiBuy { get; set; }
+    public virtual ProductVariant? ProductVariant { get; set; }
 }

LittleShop/Models/ProductVariant.cs

@@ -27,6 +27,8 @@ public class ProductVariant
 
     public ProductWeightUnit? WeightUnit { get; set; } // Optional: override product weight unit for this variant
 
+    public decimal? Price { get; set; } // Optional: override product price for this variant (if null, uses product.Price)
+
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
 
     public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;