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/src/peer/mod.rs

@@ -6,7 +6,8 @@ pub mod upload_peer;
 pub use crate::actor::peer_actor::{PeerActor, PeerMessage};
 pub use crate::actor::peer_registry::PeerRegistry;
 
-pub use download_peer::DownloadPeer;
+pub(crate) use download_peer::part_path_of;
+pub use download_peer::{DownloadError, DownloadPeer};
 
 use crate::message::Message;
 use core::fmt;