fix: do not send MDNs for hidden messages

Hidden messages are marked as seen
when chat is marked as noticed.
MDNs to such messages should not be sent
as this notifies the hidden message sender
that the chat was opened.

The issue discovered by Frank Seifferth.

src/message.rs

@@ -1934,6 +1934,7 @@ pub async fn markseen_msgs(context: &Context, msg_ids: Vec<MsgId>) -> Result<()>
             // We also don't send read receipts for contact requests.
             // Read receipts will not be sent even after accepting the chat.
             let to_id = if curr_blocked == Blocked::Not
+                && !curr_hidden
                 && curr_param.get_bool(Param::WantsMdn).unwrap_or_default()
                 && curr_param.get_cmd() == SystemMessage::Unknown
                 && context.should_send_mdns().await?