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/formState.ts

@@ -1,11 +1,11 @@
 // Helpers shared by the record form and the settings form: context
 // filtering, the save payload, 422 detail mapping and focus.
-import type { AdminRecord, ErrorBody, FormField } from '../../api/types'
+import type { AdminRecord, ErrorBody, FieldNotice, FormField } from '../../api/types'
 import { permissionValues } from './control'
-import { isMLFieldType, localeRecord } from './mlLocale'
+import { isMLField, localeRecord } from './mlLocale'
 import { isRegisteredField } from './registry'
 
-export { ML_LOCALE_CHANGE, broadcastMLLocale, isMLFieldType, localeRecord } from './mlLocale'
+export { ML_LOCALE_CHANGE, broadcastMLLocale, isMLField, isMLFieldType, localeRecord } from './mlLocale'
 
 /** The screen a field is filtered for: the two form modes and the read-only preview (D-11). */
 export type FormMode = 'create' | 'update' | 'preview'
@@ -65,7 +65,7 @@ export function editablePayload(fields: FormField[], values: AdminRecord, mode:
       out[field.name] = permissionValues(field, value)
       continue
     }
-    if (isMLFieldType(field.type)) {
+    if (isMLField(field)) {
       out[field.name] = localeRecord(value)
       continue
     }
@@ -136,7 +136,7 @@ export function presetUpdates(
   isEdited: PresetEdited,
 ): AdminRecord {
   const out: AdminRecord = {}
-  const sourceML = isMLFieldType(fields.find((field) => field.name === source)?.type)
+  const sourceML = isMLField(fields.find((field) => field.name === source))
   for (const field of fields) {
     const preset = field.preset
     if (!preset || preset.field !== source || (field.type !== 'text' && field.type !== 'mltext')) {
@@ -184,7 +184,7 @@ export function initialValues(fields: FormField[], enabledLocales: readonly stri
       out[field.name] = false
     } else if (field.type === 'relation' && field.multiple) {
       out[field.name] = []
-    } else if (isMLFieldType(field.type)) {
+    } else if (isMLField(field)) {
       const seed: Record<string, string> = {}
       for (const code of enabledLocales) {
         seed[code] = ''
@@ -225,12 +225,12 @@ export function recordDisplayName(
   values: AdminRecord,
   enabledLocales: readonly string[],
 ): string {
-  const first = fields?.find((field) => field.type === 'text' || isMLFieldType(field.type))
+  const first = fields?.find((field) => field.type === 'text' || isMLField(field))
   const value = first ? values[first.name] : undefined
   if (typeof value === 'string' || typeof value === 'number') {
     return String(value).trim()
   }
-  if (!first || !isMLFieldType(first.type)) {
+  if (!first || !isMLField(first)) {
     return ''
   }
   const texts = localeRecord(value)
@@ -246,6 +246,23 @@ export function recordDisplayName(
   return ''
 }
 
+/**
+ * A save's meta.notices (D-10, 16.2) as field name -> lines shown under the
+ * field: a notice about one content locale reads "<LOCALE>: message". Absent
+ * or empty notices give an empty map.
+ */
+export function noticeLines(notices: readonly FieldNotice[] | null | undefined): Record<string, string[]> {
+  const out: Record<string, string[]> = {}
+  for (const notice of notices ?? []) {
+    if (!notice || typeof notice.message !== 'string' || notice.message === '' || typeof notice.field !== 'string') {
+      continue
+    }
+    const line = notice.locale ? `${notice.locale.toUpperCase()}: ${notice.message}` : notice.message
+    ;(out[notice.field] ??= []).push(line)
+  }
+  return out
+}
+
 /** D-10 422 details (field -> messages) as a string-list map. */
 export function fieldErrors(details: ErrorBody['details'] | undefined): Record<string, string[]> {
   const out: Record<string, string[]> = {}