Function set_window_transparency 

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

Set window transparency/blur effects for a single window.

Targets exactly the window identified by label (from the frontend’s URL windowLabel param). Uses the window_hwnds map populated by capture_hwnd_for_label. Falls back to find_all_own_windows() only if the label’s HWND hasn’t been captured yet (e.g. very early startup).