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.

website/src/pages/docs.tsx

@@ -53,7 +53,7 @@ const PANES: readonly { num?: number; title: string; body: string }[] = [
   {
     num: 3,
     title: 'Downloads / Uploads',
-    body: 'Live transfers in both directions with progress and speed: queued, active, complete, failed.',
+    body: 'Live transfers in both directions with progress and speed: queued, active, complete, cancelled, failed. x cancels the selected transfer.',
   },
   {
     title: 'Info',
@@ -255,6 +255,8 @@ const Daemon = () => (
           { t: 'cm', text: "# opt out; open this run's own session" },
           { t: 'cmd', text: 'soulseek-rs' },
           { t: 'cm', text: '# the TUI attaches to the same session' },
+          { t: 'cmd', text: 'soulseek-rs transfer cancel <username> "<path>"' },
+          { t: 'cm', text: '# stop a download the daemon is running' },
           { t: 'cmd', text: 'soulseek-rs daemon status' },
           { t: 'cmd', text: 'soulseek-rs daemon stop' },
         ]}