feat(16.2-01): translatable plugin fields, element uploads and save notices in the admin

- MLPluginField wraps PluginField in the MLField locale switcher; the
  element gets the active content locale and that locale's string
- isMLField treats a translatable plugin field as a locale map in the form
  state, FormView adopt and the relation child modal
- summer-upload posts the file with the admin session to the field upload
  route and answers summer-upload-result {id, url} or {id, error}
- noticeLines shows meta.notices under each field as a warning status line
  (text only) and in the saved toast; the stored value is reloaded
- docs/backend/field-types.md documents the full element contract;
  modules/boardwalk/dist rebuilt

modules/boardwalk/dist/index.html

@@ -7,8 +7,8 @@
     <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-By4vm3Ku.js"></script>
-    <link rel="stylesheet" crossorigin href="./assets/index-COkYalVD.css">
+    <script type="module" crossorigin src="./assets/index-CuK_LdRk.js"></script>
+    <link rel="stylesheet" crossorigin href="./assets/index-DgNySjE5.css">
   </head>
   <body>
     <div id="app"></div>