feat(lib): surface server announcements as messages from "server"

An AdminMessage (server code 66) is how a server tells everyone it is
going down, and the client dropped it on the floor. It now arrives in
the private-message inbox as a message from the user "server", which is
also the name the server's own command replies already come from, so
every reader of that inbox shows it without a new API.

soulseek-rs-lib/src/actor/server_actor.rs

@@ -1,6 +1,7 @@
 use crate::actor::{Actor, ActorHandle, ConnectionState};
 use crate::client::ClientOperation;
 use crate::dispatcher::MessageDispatcher;
+use crate::message::server::AdminMessageHandler;
 use crate::message::server::CheckPrivilegesHandler;
 use crate::message::server::ConnectToPeerHandler;
 use crate::message::server::ExcludedSearchPhrasesHandler;
@@ -383,6 +384,7 @@ impl ServerActor {
 
         handlers.register_handler(LoginHandler);
         handlers.register_handler(ReloggedHandler);
+        handlers.register_handler(AdminMessageHandler);
         handlers.register_handler(RoomListHandler);
         handlers.register_handler(GetUserStatusHandler);
         handlers.register_handler(WatchUserHandler);

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

@@ -0,0 +1,30 @@
+//! `AdminMessage` (server code 66): an announcement from the server to
+//! everyone, surfaced as a private message from the user "server".
+
+use crate::actor::server_actor::{ServerMessage, UserMessage};
+use crate::message::{Message, MessageHandler};
+use std::sync::mpsc::Sender;
+use std::time::{SystemTime, UNIX_EPOCH};
+
+pub struct AdminMessageHandler;
+
+impl MessageHandler<ServerMessage> for AdminMessageHandler {
+    fn get_code(&self) -> u32 {
+        66
+    }
+
+    fn handle(&self, message: &mut Message, sender: Sender<ServerMessage>) {
+        let now = SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .map_or(0, |since| since.as_secs() as u32);
+        let announcement = UserMessage::new(
+            0,
+            now,
+            "server".to_string(),
+            message.read_string(),
+            false,
+        );
+        let _ =
+            sender.send(ServerMessage::PrivateMessageReceived(announcement));
+    }
+}

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

@@ -1,3 +1,4 @@
+mod admin_message;
 mod connect_to_peer;
 mod excluded_search_phrases;
 mod file_search;
@@ -19,6 +20,7 @@ mod user_left_room;
 mod watch_user;
 mod wish_list_interval;
 
+pub use admin_message::AdminMessageHandler;
 pub use connect_to_peer::ConnectToPeerHandler;
 pub use excluded_search_phrases::ExcludedSearchPhrasesHandler;
 pub use file_search::FileSearchHandler;