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/cli.rs

@@ -180,6 +180,10 @@ pub enum Commands {
     /// Download files by user and remote path
     Download(DownloadArgs),
 
+    /// Transfers in flight
+    #[command(subcommand)]
+    Transfer(TransferCommand),
+
     /// Search, pick automatically, and download in one step
     Get(GetArgs),
 
@@ -575,6 +579,18 @@ pub struct WishRunArgs {
     pub sort: SortKey,
 }
 
+#[derive(Subcommand, Debug)]
+pub enum TransferCommand {
+    /// Stop a download or upload; a cancelled download's partial file is
+    /// deleted
+    Cancel {
+        /// The other side of the transfer
+        user: String,
+        /// Remote path exactly as printed by `search`, `browse`, or `serve`
+        path: String,
+    },
+}
+
 /// Watching only lasts as long as the session doing it, so these are aimed at
 /// a running daemon: it stays connected and keeps receiving the status pushes.
 #[derive(Subcommand, Debug)]