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/Migrations/LittleShopContextModelSnapshot.cs

@@ -859,6 +859,9 @@ namespace LittleShop.Migrations
                     b.Property<Guid?>("ProductMultiBuyId")
                         .HasColumnType("TEXT");
 
+                    b.Property<Guid?>("ProductVariantId")
+                        .HasColumnType("TEXT");
+
                     b.Property<int>("Quantity")
                         .HasColumnType("INTEGER");
 
@@ -880,6 +883,8 @@ namespace LittleShop.Migrations
 
                     b.HasIndex("ProductMultiBuyId");
 
+                    b.HasIndex("ProductVariantId");
+
                     b.ToTable("OrderItems");
                 });
 
@@ -1041,6 +1046,9 @@ namespace LittleShop.Migrations
                         .HasMaxLength(100)
                         .HasColumnType("TEXT");
 
+                    b.Property<decimal?>("Price")
+                        .HasColumnType("TEXT");
+
                     b.Property<Guid>("ProductId")
                         .HasColumnType("TEXT");
 
@@ -1537,11 +1545,17 @@ namespace LittleShop.Migrations
                         .HasForeignKey("ProductMultiBuyId")
                         .OnDelete(DeleteBehavior.Restrict);
 
+                    b.HasOne("LittleShop.Models.ProductVariant", "ProductVariant")
+                        .WithMany()
+                        .HasForeignKey("ProductVariantId");
+
                     b.Navigation("Order");
 
                     b.Navigation("Product");
 
                     b.Navigation("ProductMultiBuy");
+
+                    b.Navigation("ProductVariant");
                 });
 
             modelBuilder.Entity("LittleShop.Models.Product", b =>