feat(10-03): open, edit and save a record with toast and 422 feedback

- The SPA loads the backend::lang bundle before /auth/me, sets the
  document language from meta.locale and renders plural messages with
  Intl.PluralRules; interpolate mirrors phrasebook for :name/:Name/:NAME
- Create and record routes; mapWinterUrl maps recordUrl and redirects
  onto the controller's list, create and record routes only
- List rows open their record; FormView loads the form schema and the
  record, shows context-allowed fields in the span grid, saves values
  keyed by field name and toasts the resolved saved message
- A 422 puts each message under its field (aria-invalid,
  aria-describedby), shows the plural banner, focuses the first invalid
  field in schema order and clears a field's error on change
- The D-05 registry maps text, textarea, number and dropdown; any other
  type renders the unsupported-field box with the type in DM Mono
- The admin OpenAPI document declares the write request bodies
  (AdminRecord, AdminIDsRequest) and the list filter query as a
  deepObject, so the typed client can send them
- New backend::lang form.load_failed key; boardwalk/dist rebuilt

admin/src/components/form/fields/DropdownField.vue

@@ -0,0 +1,79 @@
+<script setup lang="ts">
+import { computed } from 'vue'
+import { ChevronDown } from '@lucide/vue'
+import type { FormOption } from '../../../api/types'
+import { t } from '../../../app/i18n'
+import { controlClass, type FieldControlProps } from '../registry'
+
+const props = defineProps<FieldControlProps>()
+const emit = defineEmits<{ 'update:modelValue': [value: FormOption['value']] }>()
+
+const EMPTY = 'empty'
+const PLACEHOLDER = 'placeholder'
+
+const options = computed(() => props.field.options ?? [])
+
+function same(a: unknown, b: unknown): boolean {
+  if (a === null || a === undefined || b === null || b === undefined) {
+    return a === b || (a == null && b == null)
+  }
+  return String(a) === String(b)
+}
+
+// The <select> works on indexes so option values keep their JSON type
+// (string, number or boolean) when they are sent back.
+const selected = computed(() => {
+  const index = options.value.findIndex((option) => same(option.value, props.modelValue))
+  if (index >= 0) {
+    return String(index)
+  }
+  if (props.field.emptyOption !== undefined && (props.modelValue === null || props.modelValue === undefined || props.modelValue === '')) {
+    return EMPTY
+  }
+  return PLACEHOLDER
+})
+
+const muted = computed(() => selected.value === EMPTY || selected.value === PLACEHOLDER)
+
+function onChange(event: Event): void {
+  const value = (event.target as HTMLSelectElement).value
+  if (value === EMPTY) {
+    emit('update:modelValue', null)
+    return
+  }
+  const option = options.value[Number(value)]
+  if (option) {
+    emit('update:modelValue', option.value)
+  }
+}
+</script>
+
+<template>
+  <div class="relative">
+    <select
+      :id="controlId"
+      :name="field.name"
+      :value="selected"
+      :required="field.required || undefined"
+      :aria-required="field.required ? 'true' : undefined"
+      :aria-invalid="invalid ? 'true' : undefined"
+      :aria-describedby="describedBy || undefined"
+      :class="[controlClass(invalid), muted ? 'text-muted' : '']"
+      class="h-input appearance-none pr-10"
+      @change="onChange"
+    >
+      <option v-if="field.emptyOption !== undefined" :value="EMPTY" class="text-muted">{{ field.emptyOption }}</option>
+      <option v-else-if="selected === PLACEHOLDER" :value="PLACEHOLDER" disabled>
+        {{ t('backend::lang.form.select_placeholder') }}
+      </option>
+      <option v-for="(option, index) in options" :key="index" :value="String(index)" class="text-text">
+        {{ option.label }}
+      </option>
+    </select>
+    <ChevronDown
+      :size="16"
+      class="pointer-events-none absolute top-1/2 right-3.5 -translate-y-1/2 text-muted"
+      aria-hidden="true"
+    />
+  </div>
+</template>