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/state/usePreferences.ts

@@ -1,17 +1,22 @@
-// The signed-in admin's UI preferences: the theme (system, light or dark).
-// The server keeps them per admin (GET /auth/me preferences, PUT
-// /account/preferences); the last choice is also cached in localStorage under
-// THEME_KEY, so boot applies it before the first await and the shell never
-// renders in the wrong theme. Only the choice's code is stored; tokens and
-// record data never are (T-261016-0fm-08).
+// The signed-in admin's UI preferences: the theme (system, light or dark)
+// and the admin language. The server keeps them per admin (GET /auth/me
+// preferences, PUT /account/preferences); the last choices are also cached in
+// localStorage under THEME_KEY and LOCALE_KEY, so boot applies the theme
+// before the first await (the shell never renders in the wrong theme) and
+// loads the strings in the chosen language. Only the codes are stored;
+// tokens and record data never are (T-261016-0fm-08).
 import { readonly, ref } from 'vue'
 import { api } from '../api/client'
 import type { AdminProfile } from '../api/types'
-import { t } from '../app/i18n'
+import { currentLocale, loadStrings, t } from '../app/i18n'
 import { applyColorScheme, isThemeChoice, THEME_CHOICES, type ThemeChoice } from '../app/theme'
 import { showToast } from './useToasts'
 
 export const THEME_KEY = 'summer-admin.theme'
+export const LOCALE_KEY = 'summer-admin.locale'
+
+/** A short locale code: letters with an optional region, e.g. pl or pt-BR. */
+const LOCALE_SHAPE = /^[A-Za-z]{2,8}(?:[-_][A-Za-z0-9]{1,8})?$/
 
 const theme = ref<ThemeChoice>('system')
 let stopTheme: () => void = () => undefined
@@ -19,6 +24,11 @@ let stopTheme: () => void = () => undefined
 /** The theme currently applied. */
 export const themeChoice = readonly(theme)
 
+const localePref = ref('')
+
+/** The admin language the server stores; "" follows the request locale. */
+export const localePreference = readonly(localePref)
+
 function storage(): Storage | null {
   try {
     return typeof window === 'undefined' ? null : window.localStorage
@@ -54,6 +64,23 @@ export function cachedTheme(): ThemeChoice {
   }
 }
 
+/** The cached admin language when it is a locale-shaped code, else "". */
+export function cachedLocale(): string {
+  try {
+    const value = storage()?.getItem(LOCALE_KEY) ?? ''
+    return value.length <= 16 && LOCALE_SHAPE.test(value) ? value : ''
+  } catch {
+    return ''
+  }
+}
+
+/** Page reload indirection, so tests can stub it. */
+export const page = {
+  reload(): void {
+    window.location.reload()
+  },
+}
+
 /** Applies a theme, replacing the previous one's media subscription. */
 export function applyTheme(choice: ThemeChoice): void {
   stopTheme()
@@ -113,12 +140,47 @@ export async function applyProfilePreferences(profile: AdminProfile | null): Pro
     }
     cacheTheme(prefs.theme)
   }
+  if (typeof prefs.locale === 'string') {
+    localePref.value = prefs.locale
+    if (prefs.locale !== '' && prefs.locale !== currentLocale.value) {
+      await loadStrings(prefs.locale).catch(() => undefined)
+    }
+    cache(LOCALE_KEY, prefs.locale === '' ? null : prefs.locale)
+  }
+}
+
+/**
+ * Saves the admin language and reloads the page, so strings, navigation,
+ * settings, schemas and field types all come back in it at once (a form
+ * with unsaved changes asks first through its beforeunload prompt). The
+ * current preference does nothing; a failed save shows a danger toast.
+ */
+export async function setLocale(code: string): Promise<void> {
+  if (code === localePref.value) {
+    return
+  }
+  let ok = false
+  try {
+    const { response } = await api.PUT('/account/preferences', { body: { locale: code } })
+    ok = response.ok
+  } catch {
+    ok = false
+  }
+  if (!ok) {
+    showToast(t('backend::lang.myaccount.preferences_failed'), 'danger')
+    return
+  }
+  localePref.value = code
+  cache(LOCALE_KEY, code === '' ? null : code)
+  page.reload()
 }
 
 export function usePreferences() {
   return {
     theme: themeChoice,
+    locale: localePreference,
     setTheme,
     cycleTheme,
+    setLocale,
   }
 }