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_pay_now.dart

@@ -30,9 +30,9 @@ class MobilePayNow extends StatefulWidget {
 }
 
 class MobilePayNowState extends State<MobilePayNow> {
-  Order order;
-  List<PaymentPlatform> paymentPlatforms;
-  User _user;
+  late Order order;
+  late List<PaymentPlatform> paymentPlatforms;
+  late User _user;
 
   
   @override
@@ -74,7 +74,7 @@ class MobilePayNowState extends State<MobilePayNow> {
                       ),
                     ),
                     Text(
-                      '\$${order.totalPrice.toStringAsFixed(2)}',
+                      '\$${order.totalPrice!.toStringAsFixed(2)}',
                       style: TextStyle(
                         fontSize: 24.0,
                         fontWeight: FontWeight.bold,
@@ -91,7 +91,7 @@ class MobilePayNowState extends State<MobilePayNow> {
                     )
                 ),
               ),
-              store.state.deviceId != null && store.state.deviceId.isNotEmpty || store.state.tableNumber != null && store.state.tableNumber.isNotEmpty ?
+              store.state.deviceId != null && store.state.deviceId!.isNotEmpty || store.state.tableNumber != null && store.state.tableNumber!.isNotEmpty ?
               GestureDetector(
                 child: Container(
                   padding: EdgeInsets.only(top: 20.0, bottom: 20.0, left: 16.0, right: 16.0),