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/fields/MLPluginField.vue

@@ -0,0 +1,22 @@
+<script setup lang="ts">
+// A translatable plugin field type (D-14): MLField around PluginField. The
+// element edits the active content locale's string and receives that locale
+// in its locale attribute; the form value is the locale -> string map.
+import type { FieldControlProps } from '../control'
+import MLField from './MLField.vue'
+import PluginField from './PluginField.vue'
+
+const props = defineProps<FieldControlProps>()
+const emit = defineEmits<{ 'update:modelValue': [value: Record<string, string>] }>()
+</script>
+
+<template>
+  <MLField
+    v-bind="props"
+    :inner="PluginField"
+    :inner-type="field.type"
+    :ml-type="field.type"
+    content-locale
+    @update:model-value="emit('update:modelValue', $event)"
+  />
+</template>