fix(peer): handle UploadDenied so refused downloads fail fast

Peer message code 50 had no handler, so the dispatcher dropped it and a
download refused by the peer stayed Queued until the caller's timeout.

The new handler reads the filename and reason and routes the denial into
the same failure path as UploadFailed. A reason of "Queued" is status,
not a failure: some clients send it for a download that is still held in
their queue, and failing it would kill healthy downloads after a
reconnect, so that one is only logged.

Fixes #29

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

@@ -7,6 +7,7 @@ mod queue_upload;
 mod shared_file_list;
 mod transfer_request;
 mod transfer_response;
+mod upload_denied;
 mod upload_failed;
 
 // Re-export handlers
@@ -24,4 +25,5 @@ pub use shared_file_list::{
 };
 pub use transfer_request::TransferRequest;
 pub use transfer_response::TransferResponse;
+pub use upload_denied::UploadDeniedHandler;
 pub use upload_failed::UploadFailedHandler;