Wire targeted toast notifications to document status transitions

Adds emit_notification_user() to 7 endpoints so the acting user and
all admins/owners receive a real-time toast on key document actions:
confirm/cancel order, issue/void invoice, confirm return, confirm PO,
receive PO goods. Other staff and viewers are not notified.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/order/api/engine/cancel_order.php

@@ -23,6 +23,7 @@
         $answer['success'] = 1;
         $answer['message'] = 'Order cancelled.';
         notify_node('order_updated', [], $company_id);
+        emit_notification_user($company_id, $user_id, 'Order Cancelled', 'The sales order has been cancelled.', 'warning');
 
     } catch (PDOException $e) {
         $answer['message'] = 'Database error, please try again.';