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/formContext.ts

@@ -98,5 +98,18 @@ export const WIDGET_RESULT_EVENT = 'summer-result'
  */
 export const FIELD_CHANGE_EVENT = 'summer-change'
 
+/**
+ * Event a plugin field type's element dispatches (bubbling) to upload a file,
+ * with detail {id, file}: the element's own request id and the File. The SPA
+ * posts it with the admin session to the field upload route.
+ */
+export const FIELD_UPLOAD_EVENT = 'summer-upload'
+
+/**
+ * Event the SPA dispatches on the element when an upload ends, with detail
+ * {id, url} on success or {id, error} with a message on failure.
+ */
+export const FIELD_UPLOAD_RESULT_EVENT = 'summer-upload-result'
+
 /** How long a widget waits for its custom element to be defined. */
 export const WIDGET_TIMEOUT = 5000