Function close_window_by_label 

pub fn close_window_by_label(
    state: &Arc<AppState>,
    args: &Value,
) -> Result<Value, String>

Close a specific window by label. Used by the tear-off Phase 4 merge path: after the candidate window pulls the dragged tab into its own workspace via MoveTabToWorkspace, the dragged window is empty and should be destroyed. Posts WM_CLOSE on Win32; uses the existing UI-thread close task on other platforms.