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/persist/state.rs

@@ -9,6 +9,7 @@
 use color_eyre::Result;
 use serde::{Serialize, de::DeserializeOwned};
 use serde_json::Value;
+use soulseek_rs::{DownloadStatus, types::Download};
 use std::path::{Path, PathBuf};
 
 /// The shape every file is written in today. Bump it — and convert the older
@@ -24,6 +25,22 @@ pub struct PersistedDownload {
     pub completed: bool,
 }
 
+impl PersistedDownload {
+    #[must_use]
+    pub fn capture(download: &Download) -> Option<Self> {
+        if matches!(download.status, DownloadStatus::Cancelled) {
+            return None;
+        }
+        Some(Self {
+            username: download.username.clone(),
+            filename: download.filename.clone(),
+            size: download.size,
+            download_directory: download.download_directory.clone(),
+            completed: matches!(download.status, DownloadStatus::Completed),
+        })
+    }
+}
+
 /// One line of private-message history.
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, serde::Deserialize)]
 pub struct PersistedMessage {