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_license.dart

@@ -97,7 +97,7 @@ class MobileRenewLicenseState extends State<MobileRenewLicense> {
           ),
           autofocus: true,
           validator: (String? value) {
-            if (value.trim().isEmpty) {
+            if (value!.trim().isEmpty) {
               return S.of(context).please_enter_group_number;
             }
             return null;