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

frontend/app/test/payout_backend_test.dart

@@ -76,6 +76,14 @@ void main() {
         await t.ensureVisible(review);
         if (hasMethod) {
           expect(find.text(destinationLabel), findsOneWidget);
+          final choose = find.byKey(
+            const ValueKey('choose-configured-destination'),
+          );
+          expect(t.widget<FilledButton>(review).onPressed, isNull);
+          await t.ensureVisible(choose);
+          await t.tap(choose);
+          await t.pumpAndSettle();
+          await t.ensureVisible(review);
           await t.tap(review);
           await t.pumpAndSettle();
           expect(wallet.received, 'method-ref');