feat(tui): dismiss a finished download with d

A completed row used to ignore d: only a queued transfer could be pulled
out of the list, so a finished one sat there until the next restart. d
now goes through one handler that drops a finished row whatever the
session thinks of it — a completed transfer can live only in this window,
restored from an earlier run, where there is nothing to forget.

The finished-status test was spelled out twice in clear_finished_downloads
and had to agree by hand; both callers now share is_finished().

soulseek-rs/src/ui/main_tui/render.rs

@@ -393,7 +393,7 @@ impl MainTui {
                     vec![
                         ("p", "pause/resume"),
                         ("r", "retry failed"),
-                        ("d", "delete queued"),
+                        ("d", "delete queued/done"),
                         ("c", "clear finished"),
                         ("b", "browse user"),
                         ("1-3", "focus pane"),