feat(quick-261016-0fm): per-admin theme and locale preferences

- GET and PUT /account/preferences (own row only, interactive sessions,
  CSRF header on cookie writes) store admin_theme and admin_locale in
  backend_users.metadata, keeping every other key
- GET /auth/me and the My account answers carry preferences
- phrasebook.Translator.Locales lists the locales defining a prefix
- route tables, conformance cases, OpenAPI document, TS types and docs

admin/tests/helpers.ts

@@ -28,6 +28,7 @@ export const profile = {
     subtitle: '',
     is_superuser: false,
     role: { id: 2, code: 'developer', name: 'Developer' },
+    preferences: { theme: 'system', locale: '' },
   },
   meta: {},
 }