fix(api): order order_items so primary precedes addons in getOrderItems

The SELECT in orders_o::getOrderItems had no explicit ORDER BY clause, so
MySQL returned rows in undefined order. On the POS Fuldfør click and the
superuser invoice tree, addons (related_item_id != NULL) were sometimes
returned before their primary item, which broke the FE tree-builder and
the OrderContentTable render.

Add a stable ordering: primary items first (related_item_id IS NULL DESC),
addons grouped by their parent (related_item_id ASC), and insertion order
as the final tiebreaker (id ASC).

services/nginx/app/objects/orders_o.php

@@ -615,7 +615,12 @@ class orders_o extends db
     public function getOrderItems(int $order_id): array
     {
         global $db;
-        $sql = "SELECT * FROM order_items WHERE order_id = $order_id";
+        // Order primary items first (related_item_id IS NULL), then addons grouped by
+        // their parent (related_item_id ASC), and finally fall back to insertion order
+        // (id ASC). Without an explicit ORDER BY, MySQL is free to return rows in any
+        // order, which causes the FE tree-builder to render addons before their
+        // primary on the invoice and POS displays.
+        $sql = "SELECT * FROM order_items WHERE order_id = $order_id ORDER BY (related_item_id IS NULL) DESC, related_item_id ASC, id ASC";
         $result = $db->query($sql);
         $order_items = [];
         if ($result->num_rows > 0 && $result) {