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

admin/src/main.ts

@@ -6,7 +6,7 @@ import { loadFieldTypes } from './app/fieldTypes'
 import { loadStrings } from './app/i18n'
 import { expireSession, me } from './state/useAuth'
 import { loadNavigation } from './state/useNavigation'
-import { applyTheme, cachedTheme } from './state/usePreferences'
+import { applyTheme, cachedLocale, cachedTheme } from './state/usePreferences'
 import { loadSettings } from './state/useSettings'
 import './styles/main.css'
 
@@ -18,7 +18,9 @@ async function boot(): Promise<void> {
   // Boot order (research Pattern 7): the public strings bundle first, so the
   // login screen is translated, then the session, then navigation and the
   // plugin field types.
-  await loadStrings().catch(() => undefined)
+  // The cached admin language picks the bundle's locale; me() reloads the
+  // strings when the stored preference differs.
+  await loadStrings(cachedLocale() || undefined).catch(() => undefined)
   // A 401 here only means "not signed in"; the router guard sends the
   // visitor to the login route with the requested path as redirect.
   const user = await me().catch(() => null)