feat(admin): follow presets on mltext fields per locale

- formState: changedLocales and presetUpdates hold the preset-follow rules
  for text and mltext pairs
- FormView applies presetUpdates on create, tracks per-locale hand edits of
  ML fields and follows the active ML locale
- rebuilt modules/boardwalk/dist

admin/src/components/form/formState.ts

@@ -92,6 +92,91 @@ export function presetValue(type: string, source: unknown): string {
     .replace(/^-+|-+$/g, '')
 }
 
+/** The locales whose text differs between two locale maps: every locale on
+ * either side, a missing key counting as ''. A value that is not a map counts
+ * as an empty map. */
+export function changedLocales(previous: unknown, next: unknown): string[] {
+  const before = localeRecord(previous)
+  const after = localeRecord(next)
+  const out: string[] = []
+  for (const locale of new Set([...Object.keys(before), ...Object.keys(after)])) {
+    if ((before[locale] ?? '') !== (after[locale] ?? '')) {
+      out.push(locale)
+    }
+  }
+  return out
+}
+
+/** Reports whether the administrator edited a preset target by hand: the
+ * whole field when no locale is given, one locale of an ML target otherwise. */
+export type PresetEdited = (target: string, locale?: string) => boolean
+
+function isMLType(type: string | undefined): boolean {
+  return type === 'mltext' || type === 'mlmarkdown'
+}
+
+/**
+ * The preset targets of `source` (fields.yaml `preset`) to rewrite after the
+ * source changed from `previous` to `next`: target name -> new value. Targets
+ * are text and mltext fields; a preset on any other type is ignored.
+ *
+ * - text -> text: the target takes presetValue of the source, until edited.
+ * - ML -> text: the target takes presetValue of the source's active-locale text.
+ * - text -> ML: only the target's active locale is written; its other locales stay.
+ * - ML -> ML: per locale. Each locale whose source text changed rewrites the
+ *   same locale of the target, unless the administrator edited that locale by
+ *   hand; the target's other locales stay.
+ *
+ * A preset never writes a locale the administrator neither typed in (source)
+ * nor sees (target), so untouched locales keep the server's fallback.
+ */
+export function presetUpdates(
+  fields: FormField[],
+  source: string,
+  previous: unknown,
+  next: unknown,
+  values: AdminRecord,
+  activeLocale: string,
+  isEdited: PresetEdited,
+): AdminRecord {
+  const out: AdminRecord = {}
+  const sourceML = isMLType(fields.find((field) => field.name === source)?.type)
+  for (const field of fields) {
+    const preset = field.preset
+    if (!preset || preset.field !== source || (field.type !== 'text' && field.type !== 'mltext')) {
+      continue
+    }
+    if (field.type === 'text') {
+      if (isEdited(field.name)) {
+        continue
+      }
+      out[field.name] = presetValue(preset.type, sourceML ? (localeRecord(next)[activeLocale] ?? '') : next)
+      continue
+    }
+    if (!sourceML) {
+      if (isEdited(field.name, activeLocale)) {
+        continue
+      }
+      out[field.name] = { ...localeRecord(values[field.name]), [activeLocale]: presetValue(preset.type, next) }
+      continue
+    }
+    const texts = localeRecord(next)
+    const target = { ...localeRecord(values[field.name]) }
+    let written = false
+    for (const locale of changedLocales(previous, next)) {
+      if (isEdited(field.name, locale)) {
+        continue
+      }
+      target[locale] = presetValue(preset.type, texts[locale] ?? '')
+      written = true
+    }
+    if (written) {
+      out[field.name] = target
+    }
+  }
+  return out
+}
+
 /** Initial values of a new record: schema defaults, toggles off, no ids.
  * ML fields seed an empty string for every enabled locale. */
 export function initialValues(fields: FormField[], enabledLocales: readonly string[] = []): AdminRecord {