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

frontend/packages/sxchnge_wallet/lib/src/models/wallet.dart

@@ -1,3 +1,5 @@
+import '../destinations/models.dart';
+
 enum TransferKind { topUp, exchange, payout }
 
 enum OperationStatus { pending, completed, failed }
@@ -12,6 +14,7 @@ enum WalletError {
   unsupported,
   notFound,
   uncertain,
+  destinationChanged,
 }
 
 class WalletException implements Exception {
@@ -64,12 +67,14 @@ class TransferRequest {
     required this.amount,
     this.destination,
     this.recipient = '',
+    this.payoutDestination,
   });
   final TransferKind kind;
   final String source;
   final int amount;
   final String? destination;
   final String recipient;
+  final DestinationInput? payoutDestination;
 }
 
 class Quote {
@@ -82,11 +87,15 @@ class Quote {
     required this.expiresAt,
     required this.rateNumerator,
     required this.rateDenominator,
+    this.payoutSnapshot,
+    this.payoutTerms,
   });
   final String id;
   final TransferRequest request;
   final int fee, debit, credit, rateNumerator, rateDenominator;
   final DateTime expiresAt;
+  final DestinationDetails? payoutSnapshot;
+  final PayoutTerms? payoutTerms;
 }
 
 class WalletOperation {
@@ -116,3 +125,14 @@ class AccountProfile {
   const AccountProfile(this.locale);
   final String locale;
 }
+
+class PayoutTerms {
+  const PayoutTerms(
+    this.amount,
+    this.precision,
+    this.assetLabel,
+    this.expectedSeconds,
+  );
+  final int amount, precision, expectedSeconds;
+  final String assetLabel;
+}