Fix: HandleConfirmVariant now passes variant ID instead of null

Critical fix for £0 order bug:
- When users select a variant and click 'Add to Basket', the confirmvar: callback triggers HandleConfirmVariant
- This method was passing variantId: null to AddItem(), causing cart items to have no variant and price £0
- Now looks up selected variant by name, extracts its ID, and passes it to cart
- Added logging to track which variant is being used
- Also includes CSV variant conversion utility and sample fixed import file

TeleBot/TeleBot/Handlers/CallbackHandler.cs

@@ -1753,8 +1753,23 @@ namespace TeleBot.Handlers
             var product = await _shopService.GetProductAsync(productId.Value);
             if (product == null) return;
 
-            // Add to cart with selected variants
-            var cartItem = session.Cart.AddItem(product, quantity.Value, multiBuyId, variantId: null, selectedVariants);
+            // Find the variant by name to get its ID and price
+            Guid? variantId = null;
+            if (selectedVariants.Any() && product.Variants?.Any() == true)
+            {
+                // Use the first selected variant (for single-type products)
+                var variantName = selectedVariants.First();
+                var variant = product.Variants.FirstOrDefault(v => v.Name == variantName);
+                if (variant != null)
+                {
+                    variantId = variant.Id;
+                    _logger.LogInformation("HandleConfirmVariant: Using variant {Name} (ID: {Id}) at £{Price}",
+                        variant.Name, variant.Id, variant.Price ?? product.Price);
+                }
+            }
+
+            // Add to cart with selected variant ID
+            var cartItem = session.Cart.AddItem(product, quantity.Value, multiBuyId, variantId, selectedVariants);
 
             // Send new message with post-add prompt
             await bot.SendTextMessageAsync(