Integrate email notifications into signup and login (#38)

frontend/packages/sxchnge_auth/lib/src/models/session.dart

@@ -0,0 +1,5 @@
+/// In-memory credentials. Never include these in analytics or provider state.
+class AuthSession {
+  const AuthSession(this.userId, this.accessToken, this.refreshToken);
+  final String userId, accessToken, refreshToken;
+}