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

frontend/app/lib/main.dart

@@ -37,6 +37,7 @@ class SxchngeApp extends StatefulWidget {
     this.identityService,
     this.limitsService,
     this.walletService,
+    this.destinationService,
     this.twoFactorService,
     this.photoService = const FilePhotoService(),
     this.loginService = const DummyLoginService(),
@@ -49,6 +50,7 @@ class SxchngeApp extends StatefulWidget {
   final IdentityService? identityService;
   final LimitsService? limitsService;
   final WalletService? walletService;
+  final DestinationService? destinationService;
   final TwoFactorService? twoFactorService;
   final PhotoService photoService;
   final LoginService loginService;
@@ -89,10 +91,17 @@ class _SxchngeAppState extends State<SxchngeApp> {
               initialLocale: _locale.locale.languageCode,
               factors: _authFactors,
             ));
+  late final _destinationService =
+      widget.destinationService ??
+      (_walletService.isSimulated && _walletService is DummyWalletService
+          ? _walletService.destinations
+          : const UnavailableDestinationService());
+  late final _destinations = DestinationsProvider(_destinationService);
   late final _wallet = WalletProvider(
     _walletService,
     _analytics,
     factors: _factors,
+    destinations: _destinationService,
   );
   late final _limits =
       widget.limitsService ??
@@ -131,6 +140,7 @@ class _SxchngeAppState extends State<SxchngeApp> {
     if (customer != _walletCustomer) {
       _walletCustomer = customer;
       _wallet.reset();
+      _destinations.reset();
     }
     final path = _router.routeInformationProvider.value.uri.path;
     if (path == '/' && _previousPath != '/' && _previousPath != '/two-factor') {
@@ -145,6 +155,7 @@ class _SxchngeAppState extends State<SxchngeApp> {
     _router.dispose();
     _journeys.dispose();
     _wallet.dispose();
+    _destinations.dispose();
     if (_walletService is BackendWalletService) {
       _walletService.close();
     }
@@ -161,6 +172,7 @@ class _SxchngeAppState extends State<SxchngeApp> {
   Widget build(BuildContext context) => MultiProvider(
     providers: [
       ChangeNotifierProvider.value(value: _wallet),
+      ChangeNotifierProvider.value(value: _destinations),
       ChangeNotifierProvider.value(value: _security),
       Provider<TwoFactorService>.value(value: _factors),
       Provider<JourneyRecorder>.value(value: _journeys),