fix(client): 修复打印按钮加载后不激活问题

setState 触发重建，确保二维码加载完成后按钮变为可点击。

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

client/lib/screens/products/product_detail_screen.dart

@@ -519,7 +519,9 @@ class _QRCodeDialogState extends ConsumerState<_QRCodeDialog> {
     _future = ref
         .read(productRepositoryProvider)
         .getQRCodeBytes(widget.productId)
-      ..then((b) => _bytes = b);
+      ..then((b) {
+        if (mounted) setState(() => _bytes = b);
+      });
   }
 
   Future<void> _print() async {