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>

composer.json

@@ -11,6 +11,7 @@
         "laravel/framework": "^11.9",
         "laravel/tinker": "^2.9",
         "leandrocfe/filament-apex-charts": "^3.1",
+        "picqer/php-barcode-generator": "^3.3",
         "spatie/laravel-activitylog": "^4.9",
         "z3d0x/filament-logger": "^0.7.2"
     },