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/views/FormView.vue

@@ -24,6 +24,7 @@ import {
   fieldErrors,
   focusField,
   initialValues,
+  isMLFieldType,
   mergeMLValue,
   panelDomId,
   presetUpdates,
@@ -177,7 +178,7 @@ function adopt(record: RecordEnvelope | undefined): void {
     for (const field of fields.value) {
       if (field.type === 'password') {
         next[field.name] = ''
-      } else if (field.type === 'mltext' || field.type === 'mlmarkdown') {
+      } else if (isMLFieldType(field.type)) {
         next[field.name] = mergeMLValue(record.data[field.name], locales)
       }
     }
@@ -221,7 +222,7 @@ async function load(): Promise<void> {
 const edited = new Set<string>()
 // The locales of each ML field the administrator edited by hand.
 const editedLocales = new Map<string, Set<string>>()
-// The locale every ML control shows (MLTextField broadcasts the choice).
+// The locale every ML control shows (any ML control broadcasts the choice).
 const mlLocale = ref('en')
 
 function firstMLLocale(): string {
@@ -249,7 +250,7 @@ function update(name: string, value: unknown): void {
   const next = { ...values.value, [name]: value }
   edited.add(name)
   const type = fields.value.find((field) => field.name === name)?.type
-  if (type === 'mltext' || type === 'mlmarkdown') {
+  if (isMLFieldType(type)) {
     let locales = editedLocales.get(name)
     if (!locales) {
       locales = new Set<string>()