Integrate email notifications into signup and login (#38)

frontend/packages/sxchnge_auth/lib/src/dto/backend.dart

@@ -0,0 +1,23 @@
+import 'dart:convert';
+
+class BackendDTO {
+  BackendDTO(this.data);
+  factory BackendDTO.decode(String body) => BackendDTO(
+    (jsonDecode(body) as Map<String, dynamic>)['data'] as Map<String, dynamic>,
+  );
+  final Map<String, dynamic> data;
+  String get error => data['error'] as String? ?? 'unavailable';
+  String get userId =>
+      data['userId'] as String? ??
+      (data['account'] as Map<String, dynamic>?)?['id'] as String? ??
+      '';
+  String get pendingToken =>
+      (data['pendingToken'] as Map<String, dynamic>)['token'] as String;
+  String get target => data['target'] as String;
+  DateTime get expiresAt => DateTime.parse(data['expiresAt'] as String);
+  DateTime get resendAt => DateTime.parse(data['resendAt'] as String);
+  String get accessToken =>
+      (data['accessToken'] as Map<String, dynamic>)['token'] as String;
+  String get refreshToken =>
+      (data['refreshToken'] as Map<String, dynamic>)['token'] as String;
+}