feat(lib): send and receive private messages

Wire private messaging through the client:

- MessageFactory::build_message_user (code 22) and build_message_acked
  (code 23);
- the incoming MessageUser handler now acknowledges freshly delivered
  messages and forwards them to the client instead of only logging them;
- ClientContext buffers received UserMessages;
- Client::send_private_message and Client::take_private_messages expose the
  feature, and UserMessage (with accessors) is re-exported from the crate.

Verified end to end against soulfind: a_private_message_is_delivered_between_users
logs in two clients and asserts one receives the other's message.

soulseek-rs-lib/src/message/server/message_user.rs

@@ -1,5 +1,6 @@
 use crate::actor::server_actor::{ServerMessage, UserMessage};
 use crate::info;
+use crate::message::server::MessageFactory;
 use crate::message::{Message, MessageHandler};
 
 use std::sync::mpsc::Sender;
@@ -11,7 +12,7 @@ impl MessageHandler<ServerMessage> for MessageUser {
         22
     }
 
-    fn handle(&self, message: &mut Message, _sender: Sender<ServerMessage>) {
+    fn handle(&self, message: &mut Message, sender: Sender<ServerMessage>) {
         let id = message.read_int32();
         let timestamp = message.read_int32();
         let username = message.read_string();
@@ -27,5 +28,17 @@ impl MessageHandler<ServerMessage> for MessageUser {
 
         info!("[MessageUser] User message received:{:?}", user_message);
         user_message.print();
+
+        // Acknowledge freshly delivered messages so the server does not keep
+        // re-sending them on every reconnect.
+        if new_message {
+            let _ = sender.send(ServerMessage::SendMessage(
+                MessageFactory::build_message_acked(id),
+            ));
+        }
+
+        // Surface the message to the client so it can be read via the API.
+        let _ =
+            sender.send(ServerMessage::PrivateMessageReceived(user_message));
     }
 }