Fix UI freeze when pressing Down in an empty table

tview's table navigation never terminates when no row is selectable. Turn
selection off while the Downloads panels and the release list are empty.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/ui/downloads.go

@@ -293,10 +293,14 @@ func fill(t *tview.Table, expand int, rows [][]string) {
 // shows placeholder text when there are no rows.
 func restoreSelection(t *tview.Table, n, selRow int, placeholder string) {
 	if n == 0 {
+		// With no selectable row, tview's Down/Up handling never terminates,
+		// so turn selection off entirely.
 		t.SetCell(1, 0, tview.NewTableCell("[gray]"+placeholder).SetSelectable(false))
+		t.SetSelectable(false, false)
 		t.Select(0, 0)
 		return
 	}
+	t.SetSelectable(true, false)
 	t.Select(max(selRow, 1), 0)
 }