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/proto/dto.rs

@@ -111,6 +111,7 @@ pub enum DownloadStatusDto {
         total_bytes: u64,
     },
     Completed,
+    Cancelled,
     Failed {
         reason: Option<String>,
     },
@@ -138,6 +139,7 @@ impl From<&DownloadStatus> for DownloadStatusDto {
                 total_bytes: *total_bytes,
             },
             DownloadStatus::Completed => Self::Completed,
+            DownloadStatus::Cancelled => Self::Cancelled,
             DownloadStatus::Failed(reason) => Self::Failed {
                 reason: reason.clone(),
             },
@@ -167,6 +169,7 @@ impl From<DownloadStatusDto> for DownloadStatus {
                 total_bytes,
             },
             DownloadStatusDto::Completed => Self::Completed,
+            DownloadStatusDto::Cancelled => Self::Cancelled,
             DownloadStatusDto::Failed { reason } => Self::Failed(reason),
             DownloadStatusDto::TimedOut => Self::TimedOut,
         }