phase3: nullfix.py batch script — bang/late/param-nullable

Automated null-safety fixes driven by dart analyze (no corruption):
- unchecked_use_of_nullable_value: insert '!' on receiver (property/method/[]/op)
- not_initialized field/var: mark 'late'
- missing_default_value_for_parameter: nullable param
Errors: 2374(peak) -> 907

lib/widgets/desktop/shopping_cart_widget.dart

@@ -19,14 +19,14 @@ class ShoppingCartWidget extends StatefulWidget {
 }
 
 class ShoppingCartWidgetState extends State<ShoppingCartWidget> {
-  CartInfo cartInfo;
+  late CartInfo cartInfo;
   double totalPrice = 0.0;
 
   @override
   Widget build(BuildContext context) {
     totalPrice = 0.0;
     cartInfo = Utils.getCartInfoByBusiness(store.state.cartInfos, widget.business);
-    if (cartInfo != null && cartInfo.businessInfo.id == widget.business.id) {
+    if (cartInfo != null && cartInfo.businessInfo!.id == widget.business.id) {
       totalPrice = cartInfo.getTotalPrice();
     }
     Row row = Row(