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/api.rs

@@ -116,6 +116,7 @@ pub trait SessionApi: Send + Sync {
     fn resume_download(&self, username: &str, filename: &str) -> bool;
     fn remove_queued_download(&self, username: &str, filename: &str) -> bool;
     fn remove_download(&self, username: &str, filename: &str) -> bool;
+    fn cancel_download(&self, username: &str, filename: &str) -> bool;
 
     fn uploads(&self) -> Vec<UploadInfo>;
     fn take_upload_events(&self) -> Vec<UploadInfo>;
@@ -301,6 +302,10 @@ impl SessionApi for Client {
         Self::remove_download(self, username, filename)
     }
 
+    fn cancel_download(&self, username: &str, filename: &str) -> bool {
+        Self::cancel_download(self, username, filename)
+    }
+
     fn uploads(&self) -> Vec<UploadInfo> {
         Self::uploads(self)
     }