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/relation/RelationChildModal.vue

@@ -25,7 +25,7 @@ import {
   fieldErrors,
   focusField,
   initialValues,
-  isMLFieldType,
+  isMLField,
   mergeMLValue,
   panelDomId,
   snapshot,
@@ -218,7 +218,7 @@ function adopt(record: AdminRecord, meta?: RecordMeta): void {
   for (const field of fields.value) {
     if (field.type === 'password') {
       next[field.name] = ''
-    } else if (isMLFieldType(field.type)) {
+    } else if (isMLField(field)) {
       next[field.name] = mergeMLValue(record[field.name], locales)
     }
   }