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/releases.go

@@ -166,6 +166,8 @@ func (v *releasesView) render() {
 		}
 		v.table.SetCell(1, 1, tview.NewTableCell("[gray]"+msg).SetSelectable(false))
 	}
+	// With no selectable row tview's Down/Up handling never terminates.
+	v.table.SetSelectable(len(v.shown) > 0, false)
 	if len(v.shown) > 0 {
 		v.table.Select(max(selRow, 1), 0)
 	} else {