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/pages/contact_us.dart

@@ -22,7 +22,7 @@ class ContactUs extends StatefulWidget {
 }
 
 class ContactUsState extends State<ContactUs> {
-  Business business;
+  late Business business;
 
   @override
   Widget build(BuildContext context) {