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/api/types.ts

@@ -58,6 +58,8 @@ export type ControllerAssets = Schemas['cabana.ControllerAssets']
 export type FieldTypeAssets = Schemas['cabana.FieldTypeAssets']
 /** Answer of a plugin field type upload: the URL of the stored file. */
 export type FieldUploadResponse = Schemas['cabana.FieldUploadResponse']
+/** One save notice of a plugin field type: field, content locale and message. */
+export type FieldNotice = Schemas['cabana.FieldNotice']
 /** A registered toolbar action the admin may run, with its localized label. */
 export type ToolbarAction = Schemas['cabana.ToolbarAction']
 /** One node of a rendered partial: an allowlisted element or a text node. */