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/FormGrid.vue

@@ -10,6 +10,8 @@ defineProps<{
   fields: FormFieldSchema[]
   values: AdminRecord
   errors: Record<string, string[]>
+  /** Save notices per field (meta.notices as noticeLines); none when absent. */
+  notices?: Record<string, string[]>
   labels?: RecordMeta['labels']
   source?: ControllerParams | null
   recordId?: number | null
@@ -45,6 +47,7 @@ function spanClass(field: FormFieldSchema): string {
       :field="field"
       :model-value="values[field.name]"
       :errors="errors[field.name]"
+      :notices="notices?.[field.name]"
       :labels="labels?.[field.name]"
       :source="source"
       :record-id="recordId"