Fix: TeleBot now uses variant prices when adding to cart

CRITICAL BUG FIX: HandleAddToCart was only checking MultiBuys for price,
never ProductVariants. This caused all variant-based products to use the
base price (£0), resulting in £0 orders.

Changes:
- HandleAddToCart now checks variants FIRST for pricing
- Falls back to multi-buy, then base price (correct priority order)
- Uses proper Product-based AddItem() method to pass variant IDs
- Added logging to track which pricing method is used
- HandleQuickBuy already had correct variant detection (no changes)

Result: Orders now correctly calculate total using variant prices (e.g., £90, £160)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

TeleBot/TeleBot/Handlers/CallbackHandler.cs

@@ -479,12 +479,26 @@ namespace TeleBot.Handlers
                 return;
             }
 
-            // Get price based on multi-buy or base product
+            // Get price based on variant, multi-buy, or base product (priority order)
             decimal price = product.Price;
             string itemName = product.Name;
             int finalQuantity = quantity.Value;
+            Guid? variantId = null;
 
-            if (multiBuyId.HasValue && product.MultiBuys != null)
+            // Priority 1: Variant price (if variant selected by name - legacy string-based variants)
+            if (!string.IsNullOrEmpty(selectedVariant) && product.Variants?.Any() == true)
+            {
+                var variant = product.Variants.FirstOrDefault(v => v.Name == selectedVariant);
+                if (variant != null)
+                {
+                    price = variant.Price ?? product.Price; // Use variant price or fallback to product price
+                    variantId = variant.Id;
+                    itemName = $"{product.Name} - {variant.Name}";
+                    _logger.LogInformation("Using variant price: {Variant} at £{Price}", variant.Name, price);
+                }
+            }
+            // Priority 2: Multi-buy price
+            else if (multiBuyId.HasValue && product.MultiBuys != null)
             {
                 var multiBuy = product.MultiBuys.FirstOrDefault(mb => mb.Id == multiBuyId);
                 if (multiBuy != null)
@@ -492,16 +506,21 @@ namespace TeleBot.Handlers
                     price = multiBuy.Price;
                     itemName = $"{product.Name} ({multiBuy.Name})";
                     finalQuantity = multiBuy.Quantity; // Use multi-buy quantity
+                    _logger.LogInformation("Using multi-buy price: {MultiBuy} at £{Price}", multiBuy.Name, price);
                 }
             }
 
-            // Add variant to item name if selected
+            // Use the newer AddItem signature that properly handles variants
-            if (!string.IsNullOrEmpty(selectedVariant))
+            if (variantId.HasValue)
             {
-                itemName += $" - {selectedVariant}";
+                // Use the Product-based AddItem method for proper variant tracking
+                session.Cart.AddItem(product, finalQuantity, multiBuyId, variantId);
             }
-
+            else
+            {
+                // Legacy method for backward compatibility (no variant)
                 session.Cart.AddItem(productId.Value, itemName, price, finalQuantity, multiBuyId, selectedVariant);
+            }
 
             // Track add to cart action
             await _activityTracker.TrackActivityAsync(