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/snapshot.rs

@@ -6,7 +6,6 @@ use super::state::{PersistedDownload, PersistedMessage};
 use crate::models::{
     AppState, ChatMessage, MessageDirection, SearchEntry, SearchStatus,
 };
-use soulseek_rs::DownloadStatus;
 
 /// How much private-message history survives a restart.
 /// a flat cap on the whole log rather than per conversation; make
@@ -34,16 +33,7 @@ impl Snapshot {
         let downloads = state
             .downloads
             .iter()
-            .map(|entry| PersistedDownload {
-                username: entry.download.username.clone(),
-                filename: entry.download.filename.clone(),
-                size: entry.download.size,
-                download_directory: entry.download.download_directory.clone(),
-                completed: matches!(
-                    entry.download.status,
-                    DownloadStatus::Completed
-                ),
-            })
+            .filter_map(|entry| PersistedDownload::capture(&entry.download))
             .collect();
 
         let mut queries: Vec<String> = Vec::new();
@@ -121,6 +111,7 @@ pub fn restore_searches(state: &mut AppState, queries: &[String]) {
 mod tests {
     use super::*;
     use crate::models::DownloadEntry;
+    use soulseek_rs::DownloadStatus;
     use soulseek_rs::types::{Download, DownloadMetadata};
     use std::sync::atomic::AtomicBool;
     use std::sync::{Arc, mpsc};
@@ -188,6 +179,28 @@ mod tests {
         assert_eq!(snapshot.downloads[0].download_directory, "/music");
     }
 
+    #[test]
+    fn a_cancelled_download_is_not_captured() {
+        let mut state = AppState::new();
+        state
+            .downloads
+            .push(download("gone.mp3", DownloadStatus::Cancelled));
+        state
+            .downloads
+            .push(download("queued.mp3", DownloadStatus::Queued));
+
+        let snapshot = Snapshot::capture(&state);
+        assert_eq!(
+            snapshot
+                .downloads
+                .iter()
+                .map(|d| d.filename.as_str())
+                .collect::<Vec<_>>(),
+            vec!["queued.mp3"],
+            "a cancel must not come back as a retry after a restart"
+        );
+    }
+
     #[test]
     fn capture_takes_queries_in_order_without_duplicates() {
         let mut state = AppState::new();

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 {