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

frontend/packages/sxchnge_wallet/test/backend_test.dart

@@ -235,11 +235,52 @@ void main() {
     addTearDown(draft.dispose);
     await wallet.refresh();
     expect(draft.source, 'ledger');
-    expect(draft.recipient, 'method');
+    expect(draft.recipient, isEmpty);
     expect(draft.hasDraft, false);
+    draft.change(recipient: 'method');
+    expect(draft.hasDraft, true);
     draft.change(amount: '20.01');
     await draft.review();
     await draft.submit(approval: const FactorGrant('approved-intent-key'));
     expect(draft.result, isNotNull);
   });
+
+  test('new typed destinations cannot be silently dropped into the legacy live API', () async {
+    var calls = 0;
+    final service = BackendWalletService(
+      baseUri: Uri.parse('https://example.test'),
+      accessToken: () => 'test-token',
+      client: MockClient((request) async {
+        calls++;
+        return http.Response('{}', 500);
+      }),
+    );
+    addTearDown(service.close);
+    await expectLater(
+      service.quote(
+        const TransferRequest(
+          kind: TransferKind.payout,
+          source: 'ledger',
+          amount: 100,
+          payoutDestination: OneTimeDestination(
+            CryptoDestination(
+              assetId: 'asset',
+              networkId: 'network',
+              address: 'address',
+              assetLabel: 'Asset',
+              networkLabel: 'Network',
+            ),
+          ),
+        ),
+      ),
+      throwsA(
+        isA<WalletException>().having(
+          (e) => e.code,
+          'code',
+          WalletError.unsupported,
+        ),
+      ),
+    );
+    expect(calls, 0);
+  });
 }