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/types.rs

@@ -186,12 +186,7 @@ pub struct Download {
 impl Download {
     #[must_use]
     pub const fn is_finished(&self) -> bool {
-        matches!(
-            self.status,
-            DownloadStatus::Completed
-                | DownloadStatus::Failed(_)
-                | DownloadStatus::TimedOut
-        )
+        self.status.is_terminal()
     }
 
     #[must_use]
@@ -234,11 +229,25 @@ pub enum DownloadStatus {
         total_bytes: u64,
     },
     Completed,
+    Cancelled,
     /// Failed, optionally with a human-readable reason.
     Failed(Option<String>),
     TimedOut,
 }
 
+impl DownloadStatus {
+    #[must_use]
+    pub const fn is_terminal(&self) -> bool {
+        matches!(
+            self,
+            Self::Completed
+                | Self::Cancelled
+                | Self::Failed(_)
+                | Self::TimedOut
+        )
+    }
+}
+
 /// Whether a user is reachable, as the server reports it.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
 pub enum UserStatus {