feat(quick-261016-0fm): language switcher in the user menu

- loadStrings sends the chosen language as ?locale= and keeps
  meta.locales as availableLocales
- the user menu lists the admin locales, each named in its own language;
  choosing one saves it through PUT /account/preferences, caches it and
  reloads the page
- boot loads the strings in the cached language; me() reloads them when
  the stored preference differs

docs/backend/admin-spa.md

@@ -37,6 +37,8 @@ The user menu in the top bar has a My account item that opens `<prefix>/account`
 
 The button at the top of the plugin rail switches the theme: each click moves from system (follow the browser's `prefers-color-scheme`) to light, to dark and back to system, and its icon and label name the current choice. The choice is saved for the signed-in administrator through `PUT <prefix>/api/v1/account/preferences` and comes back as `preferences.theme` in `GET .../auth/me`, so another browser shows the same theme after sign-in. The SPA also caches the last choice in `localStorage` (`summer-admin.theme`) and applies it before anything renders. When the save fails the previous theme returns with an error message. See [My account](users-and-permissions.md#my-account).
 
+When the server offers more than one admin language (`meta.locales` of `GET .../lang`), the user menu lists them under Language, each named in its own language. Choosing one saves it as the administrator's `locale` preference, caches it in `localStorage` (`summer-admin.locale`) and reloads the page, so the strings, navigation, settings, schemas and validation messages all come back in that language, in any browser and whatever `Accept-Language` says. A form with unsaved changes asks before the reload.
+
 ## Types from OpenAPI
 
 The admin API is described by swag annotations in cabana. `scripts/check-admin-openapi.sh` generates the OpenAPI document (`admin/openapi/admin.json`) from them and the SPA's TypeScript types (`admin/src/api/schema.d.ts`) from the document, so the SPA's API client is checked against the server's shapes at compile time. Type overrides for swag live in the root `.swaggo` (it documents `json.RawMessage` as an untyped value, which swag cannot resolve on its own). `--check` fails when either committed file is out of date: