addActivity method
Implementation
Future<void> addActivity({
required String email,
required String action,
required String message,
}) async {
final users = await _readUsers();
final index = users.indexWhere((u) => u.email == email);
if (index < 0) return;
final user = users[index];
final activities = [
UserActivity(action: action, message: message, at: DateTime.now()),
...user.activities,
];
users[index] = user.copyWith(
activities: activities.take(100).toList(),
updatedAt: DateTime.now(),
);
await _writeUsers(users);
}