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-lib/examples/stress.rs

@@ -1197,6 +1197,7 @@ fn main() {
             soulseek_rs::DownloadStatus::Queued => "Queued",
             soulseek_rs::DownloadStatus::InProgress { .. } => "InProgress",
             soulseek_rs::DownloadStatus::Completed => "Completed",
+            soulseek_rs::DownloadStatus::Cancelled => "Cancelled",
             soulseek_rs::DownloadStatus::Failed(_) => "Failed",
             soulseek_rs::DownloadStatus::TimedOut => "TimedOut",
             soulseek_rs::DownloadStatus::Paused { .. } => "Paused",