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/mod.rs

@@ -469,16 +469,7 @@ fn save(
     let downloads: Vec<PersistedDownload> = session
         .get_all_downloads()
         .iter()
-        .map(|download| PersistedDownload {
-            username: download.username.clone(),
-            filename: download.filename.clone(),
-            size: download.size,
-            download_directory: download.download_directory.clone(),
-            completed: matches!(
-                download.status,
-                soulseek_rs::DownloadStatus::Completed
-            ),
-        })
+        .filter_map(PersistedDownload::capture)
         .collect();
     let _ = store.save_downloads(&downloads);