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/general/attribute/options_base.dart

@@ -16,9 +16,9 @@ abstract class OptionsBase extends StatefulWidget {
 }
 
 abstract class OptionsBaseState<Base extends OptionsBase> extends State<Base> {
-  Product product;
-  Map<String, dynamic> selections;
-  int index;
+  late Product product;
+  late Map<String, dynamic> selections;
+  late int index;
 
   final Color disabledBackgroundColor = new Color(0xFFBCBCBC);