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

frontend/packages/sxchnge_wallet/lib/src/services/backend.dart

@@ -94,10 +94,15 @@ class BackendWalletService implements WalletService {
   @override
   Future<WalletSnapshot> fetch() => _mapped(send(''), BackendMapper.snapshot);
   @override
-  Future<Quote> quote(TransferRequest request) => _mapped(
-    send('/quote', body: BackendMapper.request(request)),
-    BackendMapper.quote,
-  );
+  Future<Quote> quote(TransferRequest request) async {
+    if (request.payoutDestination != null)
+      throw const WalletException(WalletError.unsupported);
+    return _mapped(
+      send('/quote', body: BackendMapper.request(request)),
+      BackendMapper.quote,
+    );
+  }
+
   @override
   Future<WalletOperation> submit(
     String quoteId,

frontend/packages/sxchnge_wallet/lib/src/services/dummy.dart

@@ -5,6 +5,9 @@ import 'package:sxchnge_auth/sxchnge_auth.dart';
 import 'package:sxchnge_kyc/limits_transport.dart';
 import 'package:sxchnge_kyc/sxchnge_kyc.dart';
 
+import '../destinations/dummy.dart';
+import '../destinations/models.dart';
+import '../destinations/transport.dart';
 import '../models/wallet.dart';
 import '../transport/wallet.dart';
 import 'contracts.dart';
@@ -14,13 +17,17 @@ class DummyWalletService implements WalletService {
   DummyWalletService(
     this.identity, {
     this.factors,
+    DummyDestinationService? destinations,
     this.delay = const Duration(milliseconds: 350),
     this.settlement = const Duration(seconds: 2),
     DateTime Function()? now,
     String initialLocale = 'en',
-  }) : now = now ?? DateTime.now,
+  }) : destinations = destinations ?? DummyDestinationService(now: now),
+       now = now ?? DateTime.now,
        _locale = initialLocale;
   final IdentityService identity;
+  final DummyDestinationService destinations;
+  final _destinationLeases = <String, DestinationLease>{};
   final TwoFactorService? factors;
   final Duration delay, settlement;
   final DateTime Function() now;
@@ -59,6 +66,10 @@ class DummyWalletService implements WalletService {
   };
   int _eur(int minor, String id) => (minor * _weight(id) + 1099) ~/ 1100;
   void _settle() {
+    _quotes.removeWhere((_, quote) => !now().isBefore(quote.expires));
+    _destinationLeases.removeWhere(
+      (_, lease) => !now().isBefore(lease.expiresAt),
+    );
     for (final o in _submissions.values) {
       if (o.status != 'pending' || now().isBefore(o.ready)) continue;
       o.status = o.fail ? 'failed' : 'completed';
@@ -133,9 +144,19 @@ class DummyWalletService implements WalletService {
     if (r.kind == TransferKind.exchange &&
         (r.source == r.destination || r.destination == null))
       throw const WalletException(WalletError.unsupported);
-    if (r.kind == TransferKind.payout &&
-        (r.recipient.trim().length < 6 || r.recipient.length > 140))
-      throw const WalletException(WalletError.invalidRecipient);
+    DestinationLease? lease;
+    if (r.kind == TransferKind.payout) {
+      if (r.payoutDestination == null)
+        throw const WalletException(WalletError.invalidRecipient);
+      try {
+        lease = await destinations.prepare(
+          r.payoutDestination!,
+          PayoutContext(r.source, amount: r.amount),
+        );
+      } on DestinationException {
+        throw const WalletException(WalletError.destinationChanged);
+      }
+    }
     if (r.kind == TransferKind.exchange) _account(r.destination!);
     final fee = switch (r.kind) {
       TransferKind.topUp => 0,
@@ -146,19 +167,30 @@ class DummyWalletService implements WalletService {
     final d = r.kind == TransferKind.exchange ? _weight(r.destination!) : 1;
     final q = QuoteDTO(
       'quote-${++_serial}',
-      WalletMapper.request(r),
+      WalletMapper.request(r, includePayoutInput: false),
       fee,
       r.kind == TransferKind.topUp ? r.amount : r.amount + fee,
       r.amount * n ~/ d,
       now().add(const Duration(seconds: 60)),
       n,
       d,
+      payoutTerms: lease == null
+          ? null
+          : PayoutTermsDTO(r.amount, 2, switch (lease.snapshot) {
+              CryptoDestination d => d.assetLabel,
+              RussianBankDestination() => 'RUB',
+              CardDestination() => _account(r.source).currency,
+            }, settlement.inSeconds),
+      payoutSnapshot: lease == null
+          ? null
+          : DestinationMapper.details(lease.snapshot),
     );
     if (q.credit <= 0) throw const WalletException(WalletError.invalidAmount);
     final verified = await _verified();
     _settle();
     _check(q, verified);
     _quotes[q.id] = q;
+    if (lease != null) _destinationLeases[q.id] = lease;
     return WalletMapper.quote(q);
   }
 
@@ -193,6 +225,17 @@ class DummyWalletService implements WalletService {
     if (_usedQuotes.contains(q.id))
       throw const WalletException(WalletError.expiredQuote);
     _check(q, verified);
+    final lease = _destinationLeases[q.id];
+    if (q.request.kind == 'payout' && lease == null)
+      throw const WalletException(WalletError.expiredQuote);
+    if (lease != null) {
+      try {
+        destinations.accept(lease);
+      } on DestinationException {
+        throw const WalletException(WalletError.destinationChanged);
+      }
+    }
+    // No asynchronous gap between destination acceptance and financial mutation.
     _usedQuotes.add(q.id);
     if (q.request.kind != 'topUp')
       _account(q.request.source).available -= q.debit;
@@ -205,6 +248,7 @@ class DummyWalletService implements WalletService {
     );
     failNextOperation = false;
     _submissions[idempotencyKey] = o;
+    _destinationLeases.remove(q.id);
     if (loseNextResponse) {
       loseNextResponse = false;
       throw const WalletException(WalletError.uncertain);