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/formState.ts

@@ -0,0 +1,83 @@
+// 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 { isRegistered } from './registry'
+
+export type FormMode = 'create' | 'update'
+
+/** Winter `context:` semantics: no context shows the field everywhere. */
+export function contextAllows(field: FormField, mode: FormMode): boolean {
+  const context = field.context
+  if (context === undefined || context === null) {
+    return true
+  }
+  const values = Array.isArray(context) ? context : [context]
+  return values.length === 0 || values.includes(mode)
+}
+
+/**
+ * The save body: values keyed by field name for every editable field that
+ * has a value. Read-only fields and types without a renderer are never sent.
+ */
+export function editablePayload(fields: FormField[], values: AdminRecord): AdminRecord {
+  const out: AdminRecord = {}
+  for (const field of fields) {
+    if (field.readOnly || !isRegistered(field.type)) {
+      continue
+    }
+    const value = values[field.name]
+    if (value !== undefined) {
+      out[field.name] = value
+    }
+  }
+  return out
+}
+
+/** Initial values of a new record: schema defaults, toggles off, no ids. */
+export function initialValues(fields: FormField[]): AdminRecord {
+  const out: AdminRecord = {}
+  for (const field of fields) {
+    if (field.default !== undefined) {
+      out[field.name] = field.default
+    } else if (field.type === 'switch' || field.type === 'checkbox') {
+      out[field.name] = false
+    } else if (field.type === 'relation' && field.multiple) {
+      out[field.name] = []
+    }
+  }
+  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[]> = {}
+  for (const [name, value] of Object.entries(details ?? {})) {
+    const messages = Array.isArray(value)
+      ? value.filter((item): item is string => typeof item === 'string' && item !== '')
+      : typeof value === 'string' && value !== ''
+        ? [value]
+        : []
+    if (messages.length > 0) {
+      out[name] = messages
+    }
+  }
+  return out
+}
+
+/** Stable serialization for dirty checks. */
+export function snapshot(record: AdminRecord): string {
+  return JSON.stringify(Object.keys(record).sort().map((key) => [key, record[key]]))
+}
+
+/** Focuses a field's control (FormField ids are `${prefix}-${name}`). */
+export function focusField(prefix: string, name: string): boolean {
+  const element = document.getElementById(`${prefix}-${name}`)
+  if (!element) {
+    return false
+  }
+  if (!element.matches('input, select, textarea, button, [tabindex]')) {
+    element.setAttribute('tabindex', '-1')
+  }
+  element.focus()
+  return true
+}