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

frontend/packages/sxchnge_wallet/lib/src/destinations/key.dart

@@ -0,0 +1,7 @@
+import 'dart:math';
+
+/// An opaque attempt/record reference, never derived from destination details.
+String destinationKey() => List.generate(
+  16,
+  (_) => Random.secure().nextInt(256).toRadixString(16).padLeft(2, '0'),
+).join();