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,