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/mod.rs

@@ -469,16 +469,7 @@ fn save(
     let downloads: Vec<PersistedDownload> = session
         .get_all_downloads()
         .iter()
-        .map(|download| PersistedDownload {
-            username: download.username.clone(),
-            filename: download.filename.clone(),
-            size: download.size,
-            download_directory: download.download_directory.clone(),
-            completed: matches!(
-                download.status,
-                soulseek_rs::DownloadStatus::Completed
-            ),
-        })
+        .filter_map(PersistedDownload::capture)
         .collect();
     let _ = store.save_downloads(&downloads);
 

soulseek-rs/src/daemon/openrpc.rs

@@ -44,6 +44,7 @@ fn params_schema(method: Method, generator: &mut SchemaGenerator) -> Value {
         | Method::DownloadResume
         | Method::DownloadRemove
         | Method::DownloadRemoveQueued
+        | Method::DownloadCancel
         | Method::UploadCancel => schema_of::<TransferRef>(generator),
         Method::DownloadSetDir => schema_of::<DirectoryParams>(generator),
         Method::UploadSlots => schema_of::<SlotsParams>(generator),
@@ -108,6 +109,7 @@ fn result_schema(method: Method, generator: &mut SchemaGenerator) -> Value {
         | Method::DownloadResume
         | Method::DownloadRemove
         | Method::DownloadRemoveQueued
+        | Method::DownloadCancel
         | Method::DownloadSetDir
         | Method::UploadCancel
         | Method::UploadSlots
@@ -162,6 +164,9 @@ fn summary(method: Method) -> &'static str {
         Method::DownloadResume => "Resume a paused transfer.",
         Method::DownloadRemove => "Forget a transfer entirely.",
         Method::DownloadRemoveQueued => "Drop a transfer that has not started.",
+        Method::DownloadCancel => {
+            "Stop a download, queued or in progress; its partial file is deleted."
+        }
         Method::DownloadSetDir => {
             "Land future transfers in a different folder on the daemon's host."
         }

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,
         }

soulseek-rs/src/daemon/proto/mod.rs

@@ -90,6 +90,7 @@ wire_vocabulary! {
         DownloadResume => "download.resume",
         DownloadRemove => "download.remove",
         DownloadRemoveQueued => "download.remove_queued",
+        DownloadCancel => "download.cancel",
         DownloadSetDir => "download.set_dir",
         UploadList => "upload.list",
         UploadCancel => "upload.cancel",
@@ -194,6 +195,7 @@ mod tests {
                 total_bytes: 100,
             },
             DownloadStatus::Completed,
+            DownloadStatus::Cancelled,
             DownloadStatus::Failed(Some("gone".into())),
             DownloadStatus::Failed(None),
             DownloadStatus::TimedOut,

soulseek-rs/src/daemon/server/mod.rs

@@ -184,6 +184,15 @@ impl Daemon {
                     ),
                 })
             }
+            Method::DownloadCancel => {
+                let transfer: TransferRef = parse(params)?;
+                ok(Ack {
+                    ok: self.session.cancel_download(
+                        &transfer.username,
+                        &transfer.filename,
+                    ),
+                })
+            }
 
             Method::UploadList => ok(Uploads {
                 uploads: self
@@ -846,6 +855,9 @@ mod tests {
         fn cancel_upload(&self, _username: &str, _filename: &str) -> bool {
             false
         }
+        fn cancel_download(&self, _username: &str, _filename: &str) -> bool {
+            false
+        }
         fn set_upload_slots(&self, _slots: usize) {}
         fn check_privileges(&self) -> soulseek_rs::Result<()> {
             Ok(())