feat: cancel a download from the TUI, the CLI, and the daemon

Closes #54.

A download can now be cancelled whether it is queued, in progress, or
paused. Cancelled is a terminal status: the store refuses to overwrite
it, the status channel relays nothing after it, the partial file is
deleted, and the transfer thread releases its socket at its next
one-second poll instead of the 30s read deadline. A queued download that
is cancelled declines the peer's later offer with TransferResponse
"Cancelled", and an inbound file connection for it is refused. A fresh
download of the same file evicts the dead row so it is not shadowed,
and a cancelled transfer is not re-queued after a restart.

Surfaces: Client::cancel_download, download.cancel over RPC, `transfer
cancel <user> <path>` on the CLI (which also stops an upload by that
name), and x in the transfers pane, where d dismisses the row.

soulseek-rs/src/ui/panes/download_info_pane.rs

@@ -177,6 +177,9 @@ fn build_info_lines(
         }
         DownloadStatus::Paused { .. } => ("Paused".to_string(), info_style()),
         DownloadStatus::Completed => ("Completed".to_string(), success_style()),
+        DownloadStatus::Cancelled => {
+            ("Cancelled".to_string(), inactive_style())
+        }
         DownloadStatus::Failed(_) => ("Failed".to_string(), error_style()),
         DownloadStatus::TimedOut => ("Timed out".to_string(), error_style()),
     };
@@ -271,7 +274,9 @@ fn build_info_lines(
                 )));
             }
         }
-        DownloadStatus::Completed | DownloadStatus::TimedOut => {}
+        DownloadStatus::Completed
+        | DownloadStatus::Cancelled
+        | DownloadStatus::TimedOut => {}
     }
 
     lines