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/daemon/proto/mod.rs

@@ -90,6 +90,7 @@ wire_vocabulary! {
         DownloadResume => "download.resume",
         DownloadRemove => "download.remove",
         DownloadRemoveQueued => "download.remove_queued",
+        DownloadCancel => "download.cancel",
         DownloadSetDir => "download.set_dir",
         UploadList => "upload.list",
         UploadCancel => "upload.cancel",
@@ -194,6 +195,7 @@ mod tests {
                 total_bytes: 100,
             },
             DownloadStatus::Completed,
+            DownloadStatus::Cancelled,
             DownloadStatus::Failed(Some("gone".into())),
             DownloadStatus::Failed(None),
             DownloadStatus::TimedOut,