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/general/e_transfer_pay.dart

@@ -65,7 +65,7 @@ class ETransferPay extends StatelessWidget {
                             fontSize: 15, color: Colors.black54),
                       ),
                       Text(
-                        '\$${order.totalPrice.toStringAsFixed(2)}',
+                        '\$${order.totalPrice!.toStringAsFixed(2)}',
                         style: const TextStyle(
                           fontSize: 28,
                           fontWeight: FontWeight.bold,