Fix: Variant selection now accumulates across types instead of resetting

Problem:
- Selecting Size then Color would reset Size selection
- Users could never get "Add to Cart" button with multiple variant types
- Each selection created a NEW list, wiping previous choices

Root Cause:
- HandleSetVariant created: new List<string> { variantName }
- This replaced all previous selections instead of accumulating

Fix:
1. Get existing selected variants from session
2. Find the variant type of newly selected variant
3. Remove any previous selection from the SAME type (allow changing choice)
4. Add the new selection
5. Save accumulated list back to session

Example behavior now:
- Select "Red" (Color) → selectedVariants = ["Red"]
- Select "Large" (Size) → selectedVariants = ["Red", "Large"] ✅
- Select "Blue" (Color) → selectedVariants = ["Blue", "Large"] ✅
- Can now confirm when all types selected

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

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

TeleBot/TeleBot/Handlers/CallbackHandler.cs

@@ -1430,8 +1430,27 @@ namespace TeleBot.Handlers
             var product = await _shopService.GetProductAsync(productId.Value);
             if (product == null) return;
 
-            // For single item, replace selection
-            var selectedVariants = new List<string> { variantName };
+            // Get existing selections
+            var selectedVariants = session.TempData.ContainsKey("selected_variants")
+                ? session.TempData["selected_variants"] as List<string> ?? new List<string>()
+                : new List<string>();
+
+            // Find the variant type for the selected variant
+            var selectedVariantObj = product.Variants?.FirstOrDefault(v => v.Name == variantName);
+            if (selectedVariantObj != null)
+            {
+                // Remove any previous selection from the same variant type
+                var variantsOfSameType = product.Variants
+                    .Where(v => v.VariantType == selectedVariantObj.VariantType)
+                    .Select(v => v.Name)
+                    .ToList();
+
+                selectedVariants.RemoveAll(v => variantsOfSameType.Contains(v));
+
+                // Add the new selection
+                selectedVariants.Add(variantName);
+            }
+
             session.TempData["selected_variants"] = selectedVariants;
 
             var quantity = session.TempData.ContainsKey("current_quantity")