feat(18.1-01): task 4 flat reorder in the admin SPA

- ReorderView reads flat from the reorder data; ReorderBranch takes a flat
  prop and shows only move up and move down
- ArrowRight and ArrowLeft do nothing, an inside drop lands after the row
  (tree.ts flatZone, flatMove) and flatten sends parent_id null
- vitest flat cases; docs describe the flat page; rebuilt dist

modules/boardwalk/dist/index.html

@@ -7,7 +7,7 @@
     <meta name="summer-admin-base" content="__SUMMER_ADMIN_BASE__" />
     <link rel="icon" href="./favicon.ico" sizes="48x48" />
     <title>SummerCMS</title>
-    <script type="module" crossorigin src="./assets/index-CpHJmq2q.js"></script>
+    <script type="module" crossorigin src="./assets/index-Bu2-b1tB.js"></script>
     <link rel="stylesheet" crossorigin href="./assets/index-CS4sv1WG.css">
   </head>
   <body>