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/registry.ts

@@ -14,6 +14,9 @@
 // server never sends back, so it is empty on load and after every save. The
 // permissioneditor control is a value field too: an object of permission code
 // to value, labelled as a group.
+// The ML controls (mltext, mltextarea, mlmarkdown) are thin shells over one
+// locale-switching wrapper, MLField, around the text, textarea and markdown
+// controls; each holds a locale -> text map, labelled as a group.
 import type { Component } from 'vue'
 import CheckboxField from './fields/CheckboxField.vue'
 import DatepickerField from './fields/DatepickerField.vue'
@@ -30,6 +33,7 @@ import TextField from './fields/TextField.vue'
 import TextareaField from './fields/TextareaField.vue'
 import MarkdownField from './fields/MarkdownField.vue'
 import MLTextField from './fields/MLTextField.vue'
+import MLTextareaField from './fields/MLTextareaField.vue'
 import MLMarkdownField from './fields/MLMarkdownField.vue'
 import UnsupportedField from './fields/UnsupportedField.vue'
 import WidgetField from './fields/WidgetField.vue'
@@ -65,6 +69,7 @@ const renderers = new Map<string, Component>([
   ['permissioneditor', PermissionEditorField],
   ['markdown', MarkdownField],
   ['mltext', MLTextField],
+  ['mltextarea', MLTextareaField],
   ['mlmarkdown', MLMarkdownField],
 ])
 
@@ -85,7 +90,7 @@ const valueless = new Set<string>([RELATION_MANAGER, 'widget', 'partial', 'fileu
  * Types whose control is a group rather than one focusable element: the
  * visible label is a span the group points at, not a label for an input.
  */
-const groupLabelledTypes = new Set<string>(['widget', 'partial', 'fileupload', 'permissioneditor', 'mltext', 'mlmarkdown'])
+const groupLabelledTypes = new Set<string>(['widget', 'partial', 'fileupload', 'permissioneditor', 'mltext', 'mltextarea', 'mlmarkdown'])
 
 export function rendererFor(type: string): Component {
   return renderers.get(type) ?? lazyRenderers.get(type)?.() ?? UnsupportedField