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

frontend/app/lib/destinations/nickname.dart

@@ -0,0 +1,26 @@
+import 'package:flutter/material.dart';
+
+import '../generated/i18n/app_localizations.dart';
+
+class DestinationNickname extends StatelessWidget {
+  const DestinationNickname({
+    super.key,
+    required this.controller,
+    required this.onChanged,
+    this.enabled = true,
+  });
+  final TextEditingController controller;
+  final ValueChanged<String> onChanged;
+  final bool enabled;
+  @override
+  Widget build(BuildContext context) => TextField(
+    key: const ValueKey('destination-nickname'),
+    controller: controller,
+    enabled: enabled,
+    maxLength: 80,
+    decoration: InputDecoration(
+      labelText: AppLocalizations.of(context)!.dNickname,
+    ),
+    onChanged: onChanged,
+  );
+}