fix(admin): render markdown preview from server-sanitized HTML

- MarkdownField posts the source to POST /markdown/preview when Preview
  opens and again 300 ms after a change while open; stale answers dropped
- the pane binds only data.html of a 2xx answer; a refusal is a text notice
- mlmarkdown previews the active locale and follows a locale switch
- .summer-markdown style kit restores headings, lists, code and tables
- vitest coverage, forms.md and rebuilt modules/boardwalk/dist

modules/boardwalk/dist/index.html

@@ -6,8 +6,8 @@
     <meta name="robots" content="noindex, nofollow" />
     <meta name="summer-admin-base" content="__SUMMER_ADMIN_BASE__" />
     <title>SummerCMS</title>
-    <script type="module" crossorigin src="./assets/index-CRrQr2H1.js"></script>
-    <link rel="stylesheet" crossorigin href="./assets/index-Bd6s2WVD.css">
+    <script type="module" crossorigin src="./assets/index-7UEAlsFO.js"></script>
+    <link rel="stylesheet" crossorigin href="./assets/index-BUCXRqcT.css">
   </head>
   <body>
     <div id="app"></div>