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

admin/src/components/form/registry.ts

@@ -18,8 +18,9 @@
 // locale-switching wrapper, MLField, around the text, textarea and markdown
 // controls; each holds a locale -> text map, labelled as a group.
 // Phase 16.2 adds plugin field types (D-13): a field whose schema carries an
-// `element` renders PluginField whatever its type alias, holds a form value
-// and is labelled as a group. Look such fields up with rendererForField and
+// `element` renders PluginField whatever its type alias (MLPluginField, the
+// MLField locale switcher around it, when the type is translatable), holds a
+// form value and is labelled as a group. Look such fields up with rendererForField and
 // isRegisteredField, which know the field, not only its type.
 import type { Component } from 'vue'
 import type { FormField } from '../../api/types'
@@ -30,6 +31,7 @@ import FileuploadField from './fields/FileuploadField.vue'
 import NumberField from './fields/NumberField.vue'
 import PartialField from './fields/PartialField.vue'
 import PasswordField from './fields/PasswordField.vue'
+import MLPluginField from './fields/MLPluginField.vue'
 import PluginField from './fields/PluginField.vue'
 import PermissionEditorField from './fields/PermissionEditorField.vue'
 import RelationManager from '../relation/RelationManager.vue'
@@ -115,7 +117,7 @@ export function isPluginField(field: Pick<FormField, 'element'>): boolean {
 /** The control of one field: a plugin field type's host, else rendererFor its type. */
 export function rendererForField(field: FormField): Component {
   if (isPluginField(field)) {
-    return PluginField
+    return field.translatable ? MLPluginField : PluginField
   }
   return rendererFor(field.type)
 }