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/mobile/mobile_renew_minioffice.dart

@@ -104,7 +104,7 @@ class MobileRenewMiniOfficeState extends State<MobileRenewMiniOffice> {
     );
   }
 
-  Widget buildLine(String name, String value, {double nameSize, double valueSize}) {
+  Widget buildLine(String name, String value, {double? nameSize, double? valueSize}) {
     Row row = Row(
       mainAxisAlignment: MainAxisAlignment.start,
       crossAxisAlignment: CrossAxisAlignment.start,