feat: Phase 1 — remove Stripe, switch to e-Transfer

- Drop stripe_payment + stripe_sdk deps; delete stripe pay pages/widgets
- New ETransferPay widget: shows payment@wisetronic.com + R#<orderId> remark
- goPayment stripe/native branches now route to e-Transfer
- pay_now widgets: strip saved-cards section & dead nativePay logic
- my_cards pages rewritten as e-Transfer info (route kept)
- models/utils/constants cleaned of stripe; env constraint -> Dart 3

See UPGRADE_NOTES.md. (Null-safety migration is Phase 3; deps upgrade Phase 2.)

lib/pages/stripe_pay_web.dart

@@ -1,34 +0,0 @@
-
-import 'package:flutter/material.dart';
-import 'package:responsive_builder/responsive_builder.dart';
-
-import '../models/order.dart';
-import '../models/payment_platform.dart';
-import '../models/stripe_payment_method.dart';
-import '../store/actions.dart';
-import '../store/store.dart';
-import '../widgets/desktop/desktop_stripe_pay_web.dart';
-import '../widgets/mobile/mobile_stripe_pay_web.dart';
-
-class StripePayWeb extends StatelessWidget {
-  final Order order;
-  final PaymentPlatform paymentPlatform;
-  final StripePaymentMethod stripePaymentMethod;
-
-  const StripePayWeb(this.order, this.paymentPlatform, {this.stripePaymentMethod});
-
-  @override
-  Widget build(BuildContext context) {
-    store.dispatch(UpdateContext(context));
-
-    return ResponsiveBuilder(
-      builder: (context, sizingInformation) =>
-          ScreenTypeLayout(
-            mobile: MobileStripePayWeb(order, paymentPlatform, stripePaymentMethod: stripePaymentMethod,),
-            tablet: DesktopStripePayWeb(order, paymentPlatform, stripePaymentMethod: stripePaymentMethod,),
-            desktop: DesktopStripePayWeb(order, paymentPlatform, stripePaymentMethod: stripePaymentMethod,),
-          ),
-    );
-  }
-
-}