isHalfRead method
Indicates whether the provided ChatItem was read only partially by some
User other than me.
Implementation
bool isHalfRead(ChatItem item, UserId? me) {
  return members.any((e) {
    if (e.user.id == me) {
      return false;
    }
    final LastChatRead? read = lastReads.firstWhereOrNull(
      (m) => m.memberId == e.user.id,
    );
    return read == null || read.at.isBefore(item.at);
  });
}