Add SKU/GTIN fields, live barcode preview, and Shopify price import

- Add SKU and GTIN fields to product variant repeater
- Render EAN/UPC barcode live from GTIN input (picqer/php-barcode-generator)
- Pull variant price from Shopify API instead of defaulting to 0
- Sync sku/gtin through EditProduct and CreateProduct save logic

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Filament/User/Resources/ProductResource/Pages/EditProduct.php

@@ -27,7 +27,12 @@ class EditProduct extends EditRecord
         $sizes = collect($this->data['sizes'] ?? [])
             ->mapWithKeys(function ($size) {
                 return [
-                    $size['packing_id'] => ['description' => $size['description'], 'price' =>$size['price']],
+                    $size['packing_id'] => [
+                        'description' => $size['description'] ?? null,
+                        'price'       => $size['price'],
+                        'sku'         => $size['sku'] ?? null,
+                        'gtin'        => $size['gtin'] ?? null,
+                    ],
                 ];
             })
             ->toArray();