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/ui/styles.rs

@@ -152,6 +152,7 @@ pub fn download_status_glyph(status: &DownloadStatus) -> (&'static str, Style) {
         DownloadStatus::InProgress { .. } => (GLYPH_ACTIVE, accent_style()),
         DownloadStatus::Paused { .. } => (GLYPH_PAUSED, info_style()),
         DownloadStatus::Completed => (GLYPH_DONE, success_style()),
+        DownloadStatus::Cancelled => (GLYPH_FAILED, inactive_style()),
         DownloadStatus::Failed(_) => (GLYPH_FAILED, error_style()),
         DownloadStatus::TimedOut => (GLYPH_TIMED_OUT, error_style()),
     }
@@ -237,6 +238,7 @@ mod tests {
             (DownloadStatus::Completed, GLYPH_DONE, PHOSPHOR),
             (DownloadStatus::Failed(None), GLYPH_FAILED, ALARM),
             (DownloadStatus::TimedOut, GLYPH_TIMED_OUT, ALARM),
+            (DownloadStatus::Cancelled, GLYPH_FAILED, COLOR_INACTIVE),
         ] {
             let (got_glyph, style) = download_status_glyph(&status);
             assert_eq!(got_glyph, glyph);