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/downloads_pane.rs

@@ -81,6 +81,7 @@ pub fn render_downloads_pane(
                     )
                 }
                 DownloadStatus::Completed => "Completed".to_string(),
+                DownloadStatus::Cancelled => "Cancelled".to_string(),
                 DownloadStatus::Failed(_) => "Failed".to_string(),
                 DownloadStatus::TimedOut => "Timed out".to_string(),
             };
@@ -97,7 +98,9 @@ pub fn render_downloads_pane(
                 DownloadStatus::Failed(_) | DownloadStatus::TimedOut => {
                     error_style()
                 }
-                DownloadStatus::Queued => inactive_style(),
+                DownloadStatus::Queued | DownloadStatus::Cancelled => {
+                    inactive_style()
+                }
                 _ => primary_style(),
             };