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/server/mod.rs

@@ -184,6 +184,15 @@ impl Daemon {
                     ),
                 })
             }
+            Method::DownloadCancel => {
+                let transfer: TransferRef = parse(params)?;
+                ok(Ack {
+                    ok: self.session.cancel_download(
+                        &transfer.username,
+                        &transfer.filename,
+                    ),
+                })
+            }
 
             Method::UploadList => ok(Uploads {
                 uploads: self
@@ -846,6 +855,9 @@ mod tests {
         fn cancel_upload(&self, _username: &str, _filename: &str) -> bool {
             false
         }
+        fn cancel_download(&self, _username: &str, _filename: &str) -> bool {
+            false
+        }
         fn set_upload_slots(&self, _slots: usize) {}
         fn check_privileges(&self) -> soulseek_rs::Result<()> {
             Ok(())