Build exchange workspace and shared transaction services (#28)

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

@@ -0,0 +1,13 @@
+import '../models/wallet.dart';
+
+abstract interface class WalletService {
+  bool get isSimulated;
+  Future<WalletSnapshot> fetch();
+  Future<Quote> quote(TransferRequest request);
+
+  /// Same key must reconcile uncertain submissions without duplicate execution.
+  Future<WalletOperation> submit(String quoteId, String idempotencyKey);
+  Future<WalletOperation?> findSubmission(String idempotencyKey);
+  Future<AccountProfile> profile();
+  Future<AccountProfile> saveLocale(String locale);
+}