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/Areas/Admin/Views/Products/CreateVariant.cshtml

@@ -55,6 +55,16 @@
                         <small class="form-text text-muted">Track inventory for this specific variant (optional)</small>
                     </div>
 
+                    <div class="mb-3">
+                        <label asp-for="Price" class="form-label"></label>
+                        <div class="input-group">
+                            <span class="input-group-text">£</span>
+                            <input asp-for="Price" type="number" step="0.01" class="form-control" min="0.01" placeholder="@product?.Price.ToString("F2")" />
+                        </div>
+                        <span asp-validation-for="Price" class="text-danger"></span>
+                        <small class="form-text text-muted">Override base price for this variant (optional - defaults to £@product?.Price.ToString("F2"))</small>
+                    </div>
+
                     <div class="mb-3">
                         <label asp-for="SortOrder" class="form-label"></label>
                         <input asp-for="SortOrder" type="number" class="form-control" />

LittleShop/Areas/Admin/Views/Products/EditVariant.cshtml

@@ -51,6 +51,16 @@
                         <span asp-validation-for="StockLevel" class="text-danger"></span>
                     </div>
 
+                    <div class="mb-3">
+                        <label asp-for="Price" class="form-label"></label>
+                        <div class="input-group">
+                            <span class="input-group-text">£</span>
+                            <input asp-for="Price" type="number" step="0.01" class="form-control" min="0.01" placeholder="@product?.Price.ToString("F2")" />
+                        </div>
+                        <span asp-validation-for="Price" class="text-danger"></span>
+                        <small class="form-text text-muted">Override base price (optional - defaults to £@product?.Price.ToString("F2"))</small>
+                    </div>
+
                     <div class="mb-3">
                         <label asp-for="SortOrder" class="form-label"></label>
                         <input asp-for="SortOrder" type="number" class="form-control" />

LittleShop/Areas/Admin/Views/Products/ProductVariants.cshtml

@@ -33,6 +33,7 @@
                         <tr>
                             <th>Name</th>
                             <th>Type</th>
+                            <th>Price</th>
                             <th>Stock Level</th>
                             <th>Sort Order</th>
                             <th>Status</th>
@@ -49,6 +50,16 @@
                                 <td>
                                     <span class="badge bg-info">@variant.VariantType</span>
                                 </td>
+                                <td>
+                                    @if (variant.Price.HasValue)
+                                    {
+                                        <strong>£@variant.Price.Value.ToString("F2")</strong>
+                                    }
+                                    else
+                                    {
+                                        <span class="text-muted">£@product?.Price.ToString("F2") (base)</span>
+                                    }
+                                </td>
                                 <td>
                                     @if (variant.StockLevel > 0)
                                     {