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_buy_service.dart

@@ -20,7 +20,7 @@ class MobileBuyService extends StatefulWidget {
 
 class MobileBuyServiceState extends State<MobileBuyService> {
   List<KeyValue> plans = [];
-  KeyValue selectedPlan;
+  late KeyValue selectedPlan;
   double price = 0.0;
   double tax = 0.0;
   double paymentAmount = 0.0;