chore(admin): stop tracking SPA build; add native product meta box

Remove assets/admin/build from version control (local build only going
forward). Add AKSRT_Product_Meta_Box so product price/stock/shipping/after-
sale fields can be edited on the native aksrt_product edit screen without
the SPA product drawer.

functions.php

@@ -294,6 +294,11 @@ if (is_admin() && class_exists('AKSRT_Admin_Shop_Controller')) {
     AKSRT_Admin_Shop_Controller::get_instance()->init_product_admin_columns();
 }
 
+// 商品原生编辑 meta box（价格/库存/发货售后）
+if (is_admin() && class_exists('AKSRT_Product_Meta_Box')) {
+    AKSRT_Product_Meta_Box::get_instance()->init();
+}
+
 // 商品评价/评分系统
 if (class_exists('AKSRT_Product_Review')) {
     AKSRT_Product_Review::get_instance()->init();