ver. 0.303: Fix attribute display collision + product preview button

Fix: product attributes with the same sort order value were overwriting
each other in getProductAttributes(), causing only one attribute to
display on the frontend. Now uses usort() with sequential keys.

New: Preview button in product edit form opens product page in new tab.

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

autoload/admin/Controllers/ShopProductController.php

@@ -547,6 +547,11 @@ class ShopProductController
             FormAction::cancel( $backUrl ),
         ];
 
+        if ( $productId > 0 ) {
+            $previewUrl = $this->repository->getProductUrl( $productId );
+            $actions[] = FormAction::preview( $previewUrl );
+        }
+
         return new FormEditViewModel(
             'product-edit',
             $title,