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/tests/e2e.rs

@@ -859,6 +859,18 @@ fn an_upload_failed_reply_fails_the_download_quickly() {
     });
 }
 
+#[test]
+fn an_upload_denied_reply_fails_the_download_quickly() {
+    refusal_fails_the_download_quickly("e2e_denying_peer", |filename| {
+        let mut refusal = Message::new();
+        refusal
+            .write_int32(50)
+            .write_string(filename)
+            .write_string("File not shared.");
+        refusal
+    });
+}
+
 #[test]
 fn an_interrupted_download_resumes_from_its_partial_file() {
     let server = server_or_skip!();