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/commands/transfer.rs

@@ -553,6 +553,9 @@ fn download_one(
                     reason.unwrap_or_else(|| "the transfer failed".to_string()),
                 ));
             }
+            Ok(DownloadStatus::Cancelled) => {
+                return Err(CliError::transfer("the transfer was cancelled"));
+            }
             Ok(DownloadStatus::Queued) => {
                 out.status(&format!(
                     "queued behind {}'s uploads",
@@ -585,6 +588,19 @@ fn download_one(
     }
 }
 
+pub fn cancel(ctx: &Ctx, user: &str, path: &str) -> CliResult {
+    let session = Session::open(ctx)?;
+    if session.client.cancel_download(user, path)
+        | session.client.cancel_upload(user, path)
+    {
+        ctx.out.status(&format!("cancelled {path} ({user})"));
+        return Ok(());
+    }
+    Err(CliError::no_results(format!(
+        "no download or upload of {path} with {user}"
+    )))
+}
+
 /// Parse `search`/`browse` output back into download requests.
 ///
 /// Both shapes this program emits are accepted: a JSON object per line, or