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/message/server/message_factory.rs

@@ -249,6 +249,16 @@ impl MessageFactory {
             .write_bool(true)
             .clone()
     }
+    #[must_use]
+    pub fn build_transfer_denial_message(token: u32, reason: &str) -> Message {
+        Message::new()
+            .write_int32(41)
+            .write_int32(token)
+            .write_bool(false)
+            .write_string(reason)
+            .clone()
+    }
+
     #[must_use]
     pub fn build_pierce_firewall_message(token: u32) -> Message {
         Message::new()
@@ -321,6 +331,18 @@ fn test_build_upload_transfer_request() {
     assert_eq!(transfer.size, 4096);
 }
 
+#[test]
+fn a_transfer_denial_carries_the_token_and_the_reason() {
+    let message =
+        MessageFactory::build_transfer_denial_message(555, "Cancelled");
+    let mut decoded = Message::new_with_data(message.get_buffer());
+    assert_eq!(decoded.get_message_code_u32(), 41);
+    decoded.set_pointer(8);
+    assert_eq!(decoded.read_int32(), 555);
+    assert!(!decoded.read_bool());
+    assert_eq!(decoded.read_string(), "Cancelled");
+}
+
 #[test]
 fn test_build_peer_init_message() {
     use crate::peer::ConnectionType;