Add saved payout destination UX and service contracts (#54)

frontend/app/test/workspace_test.dart

@@ -11,6 +11,8 @@ import 'package:sxchnge_wallet/sxchnge_wallet.dart';
 import 'package:sxchnge_app/main.dart';
 import 'package:sxchnge_app/providers/locale.dart';
 
+import 'destination_helpers.dart';
+
 Future<void> tap(WidgetTester t, Finder f) async {
   await t.ensureVisible(f);
   await t.tap(f);
@@ -32,6 +34,7 @@ Future<DummyWalletService> open(
     settlement: settlement,
     initialLocale: locale,
   );
+  await seedDestination(service.destinations);
   await t.pumpWidget(
     SxchngeApp(
       identityService: identity,
@@ -78,10 +81,7 @@ void main() {
     expect(find.byKey(const ValueKey('operation-completed')), findsOneWidget);
     await tap(t, find.text('Pay out').last);
     await t.enterText(find.byKey(const ValueKey('transfer-amount')), '5');
-    await t.enterText(
-      find.byKey(const ValueKey('transfer-recipient')),
-      'Sample recipient',
-    );
+    await selectPayout(t);
     await tap(t, find.byKey(const ValueKey('transfer-review')));
     expect(find.text('Recipient receives'), findsOneWidget);
     await tap(t, find.byKey(const ValueKey('transfer-confirm')));
@@ -101,10 +101,7 @@ void main() {
       );
       s.failNextOperation = true;
       await t.enterText(find.byKey(const ValueKey('transfer-amount')), '10');
-      await t.enterText(
-        find.byKey(const ValueKey('transfer-recipient')),
-        'Sample recipient',
-      );
+      await selectPayout(t);
       await tap(t, find.byKey(const ValueKey('transfer-review')));
       await tap(t, find.byKey(const ValueKey('transfer-confirm')));
       // Settlement is service-owned; refresh after the simulated processing period.