Fix: Populate existing variant panels with imported ProductVariant data

**Previous Approach (WRONG):**
- Created a separate table section to display ProductVariants
- User wanted data in the EXISTING collapsible panels, not a new section

**Proper Fix:**
- ProductImportService creates records in ProductVariants table
- Edit page's collapsible panels read from VariantsJson field (different system)
- Solution: Convert ProductVariants → VariantsJson format on page load

**Changes:**

1. **ProductsController.cs (Lines 105-115):**
   - Load ProductVariants from database
   - If VariantsJson empty but ProductVariants exist, convert them
   - Format: `[{Weight: "28g", Price: 700, StockQty: 100}, ...]`
   - JavaScript reads Price and StockQty to populate collapsible panel fields

2. **Edit.cshtml:**
   - Removed the extra table section
   - Existing collapsible panels now display imported data automatically

**Result:**
When you open Edit page, expand "Product Variants" → "Price, Stock & Weight Details",
the fields will be PRE-FILLED with your imported values (700, 100, etc.)

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

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

LittleShop/Areas/Admin/Controllers/ProductsController.cs

@@ -98,10 +98,22 @@ public class ProductsController : Controller
         var productMultiBuys = await _productService.GetProductMultiBuysAsync(id);
         ViewData["ProductMultiBuys"] = productMultiBuys;
 
-        // Load product variants
+        // Load product variants and convert to VariantsJson format for UI
         var productVariants = await _productService.GetProductVariantsAsync(id);
         ViewData["ProductVariants"] = productVariants;
 
+        // If product has no VariantsJson but has ProductVariant records, convert them
+        if ((string.IsNullOrEmpty(product.VariantsJson) || product.VariantsJson == "[]") && productVariants.Any())
+        {
+            var variantsForJson = productVariants.Select(v => new
+            {
+                Weight = v.Name, // Use variant name as the "Weight" property
+                Price = v.Price,
+                StockQty = v.StockLevel
+            }).ToList();
+            product.VariantsJson = System.Text.Json.JsonSerializer.Serialize(variantsForJson);
+        }
+
         // TODO: Add ReviewService injection and retrieve actual reviews
         // For now, providing mock review data for demonstration
         ViewData["ProductReviews"] = new[]
@@ -139,7 +151,7 @@ public class ProductsController : Controller
             StockQuantity = product.StockQuantity,
             CategoryId = product.CategoryId,
             VariantCollectionId = product.VariantCollectionId,
-            VariantsJson = product.VariantsJson,
+            VariantsJson = product.VariantsJson, // Now contains converted ProductVariants data
             IsActive = product.IsActive
         };