feat(admin): render mltextarea fields per locale

- MLField.vue is the one locale-switching wrapper; MLTextField,
  MLMarkdownField and the new MLTextareaField are shells around it
- isMLFieldType replaces the inline ML type-name checks in formState,
  FormView and RelationChildModal
- register mltextarea as a group-labelled control; rebuild dist

admin/src/components/form/mlLocale.ts

@@ -1,3 +1,11 @@
+/** The multilingual field types: their value is a locale -> text map. */
+export const ML_FIELD_TYPES: ReadonlySet<string> = new Set(['mltext', 'mltextarea', 'mlmarkdown'])
+
+/** Reports whether a field type holds a locale -> text map. */
+export function isMLFieldType(type: string | undefined): boolean {
+  return type !== undefined && ML_FIELD_TYPES.has(type)
+}
+
 /** Window event that keeps every ML control on one active locale. */
 export const ML_LOCALE_CHANGE = 'summer:ml-locale'