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);