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-lib/src/client/downloads.rs

@@ -3,6 +3,7 @@ use super::{
     Receiver, Result, RwLock, RwLockExt, Sender, error, info, mpsc,
     next_download_token,
 };
+use crate::peer::part_path_of;
 
 impl Client {
     #[must_use]
@@ -35,6 +36,24 @@ impl Client {
         }
     }
 
+    #[must_use]
+    pub fn cancel_download(&self, username: &str, filename: &str) -> bool {
+        let cancelled = match self.context.write_safe() {
+            Ok(mut ctx) => ctx.downloads.cancel_by_file(username, filename),
+            Err(e) => {
+                error!("[client] cancel_download: {}", e);
+                None
+            }
+        };
+        let Some(download) = cancelled else {
+            return false;
+        };
+        if let Some(part) = part_path_of(&download) {
+            let _ = std::fs::remove_file(part);
+        }
+        true
+    }
+
     #[must_use]
     pub fn remove_queued_download(
         &self,