Function set_window_opacity 

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

Set opacity on exactly one window by label.

Routes through the host reducer (HostCommand::SetWindowOpacity) so the change is auditable. The reducer emits WindowOpacityApplied; host_dispatch reads the event and calls the Win32 helper directly (Win32 window-style ops are safe from any thread). Replaces the global set_window_transparency path for per-window calls.