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.

assets/admin/build/index.html

@@ -1,16 +0,0 @@
-<!DOCTYPE html>
-<html lang="zh-CN">
-<head>
-  <meta charset="UTF-8" />
-  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
-  <title>AKSRT 主题设置</title>
-  <script type="module" crossorigin src="./index.js"></script>
-  <link rel="modulepreload" crossorigin href="./vendor-vue.Bykizn0M.js">
-  <link rel="modulepreload" crossorigin href="./vendor-misc.1adHN6i_.js">
-  <link rel="modulepreload" crossorigin href="./vendor-naive.BuHWlEgf.js">
-  <link rel="stylesheet" crossorigin href="./index.css">
-</head>
-<body>
-  <div id="aksrt-admin-app"></div>␍
-</body>
-</html>