feat(tui): surface the failure reason on failed downloads

Carry a human-readable reason on DownloadStatus::Failed so it flows
through the existing status channel to the TUI. The download Info pane
now shows an Error line for a failed download (connect failure, peer
disconnect, truncated transfer, peer-reported upload failure, or the
underlying transfer error). Failures without a known reason render as
before.

soulseek-rs/src/ui/panes/downloads_pane.rs

@@ -54,7 +54,7 @@ pub fn render_downloads_pane(
                 DownloadStatus::InProgress { .. } => ("⧗", warning_style()),
                 DownloadStatus::Paused { .. } => ("⏸", info_style()),
                 DownloadStatus::Completed => ("✓", success_style()),
-                DownloadStatus::Failed => ("✗", error_style()),
+                DownloadStatus::Failed(_) => ("✗", error_style()),
                 DownloadStatus::TimedOut => ("⏱", error_style()),
             };
 
@@ -91,7 +91,7 @@ pub fn render_downloads_pane(
                     )
                 }
                 DownloadStatus::Completed => "Completed".to_string(),
-                DownloadStatus::Failed => "Failed".to_string(),
+                DownloadStatus::Failed(_) => "Failed".to_string(),
                 DownloadStatus::TimedOut => "Timed out".to_string(),
             };