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/CreateProduct.php

@@ -17,10 +17,12 @@ class CreateProduct extends CreateRecord
 
         foreach ($data['sizes'] as $size) {
             ProductSize::create([
-                'product_id' => $record->id,
-                'packing_id' => $size['packing_id'],
-                'description' => $size['description'],
-                'price' => $size['price'],
+                'product_id'  => $record->id,
+                'packing_id'  => $size['packing_id'],
+                'description' => $size['description'] ?? null,
+                'price'       => $size['price'],
+                'sku'         => $size['sku'] ?? null,
+                'gtin'        => $size['gtin'] ?? null,
             ]);
         }
         return $record;