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/components/shell/UserMenu.vue

@@ -4,20 +4,27 @@ import { useRouter } from 'vue-router'
 import {
   DropdownMenuContent,
   DropdownMenuItem,
+  DropdownMenuItemIndicator,
+  DropdownMenuLabel,
   DropdownMenuPortal,
+  DropdownMenuRadioGroup,
+  DropdownMenuRadioItem,
   DropdownMenuRoot,
   DropdownMenuSeparator,
   DropdownMenuTrigger,
 } from 'reka-ui'
-import { ChevronDown, LogOut, UserRound } from '@lucide/vue'
-import { t } from '../../app/i18n'
+import { Check, ChevronDown, LogOut, UserRound } from '@lucide/vue'
+import { availableLocales, currentLocale, t } from '../../app/i18n'
 import { currentUser, logout, profileInitials } from '../../state/useAuth'
+import { localePreference, setLocale } from '../../state/usePreferences'
 
 // User menu (design: Top header): a 44px button with the avatar image (the
 // initials when the admin has none), the name above the role from /auth/me
 // (both hidden at tablet width) and a chevron; the 280px menu (Reka
 // DropdownMenu, role menu) shows the name and e-mail, a My account item that
-// opens /account and a Wyloguj item. Logging out always ends on the login
+// opens /account, a Language radio group (when the server offers more than
+// one admin locale; choosing one saves it and reloads the page) and a
+// Wyloguj item. Logging out always ends on the login
 // screen, even when the logout call fails (T-10-23).
 const router = useRouter()
 
@@ -38,6 +45,30 @@ const avatarSrc = computed(() => currentUser.value?.avatar?.thumb_url || current
 
 const roleName = computed(() => currentUser.value?.role?.name ?? '')
 
+/** The checked language: the stored preference, else the bundle locale. */
+const selectedLocale = computed(() => localePreference.value || currentLocale.value)
+
+/** A language named in itself (polski, English), else its upper-cased code. */
+function languageName(code: string): string {
+  try {
+    const name = new Intl.DisplayNames([code], { type: 'language' }).of(code)
+    if (name && name !== code) {
+      return name.charAt(0).toLocaleUpperCase(code) + name.slice(1)
+    }
+  } catch {
+    // An invalid tag falls through to the code.
+  }
+  return code.toUpperCase()
+}
+
+const languages = computed(() => availableLocales.value.map((code) => ({ code, name: languageName(code) })))
+
+function onLocale(value: unknown): void {
+  if (typeof value === 'string' && value !== '') {
+    void setLocale(value)
+  }
+}
+
 async function onAccount(): Promise<void> {
   await router.push({ name: 'account' }).catch(() => undefined)
 }
@@ -111,6 +142,29 @@ async function onLogout(): Promise<void> {
           <UserRound :size="18" aria-hidden="true" />
           <span>{{ t('backend::lang.myaccount.menu_label') }}</span>
         </DropdownMenuItem>
+        <template v-if="languages.length > 1">
+          <DropdownMenuSeparator class="my-1 h-px bg-border" />
+          <DropdownMenuLabel class="px-3 pt-1 pb-1 text-[12px] font-semibold text-muted" data-language-label>{{
+            t('backend::lang.nav.language')
+          }}</DropdownMenuLabel>
+          <DropdownMenuRadioGroup :model-value="selectedLocale" @update:model-value="onLocale">
+            <DropdownMenuRadioItem
+              v-for="language in languages"
+              :key="language.code"
+              :value="language.code"
+              :data-locale-option="language.code"
+              :lang="language.code"
+              class="flex h-10 cursor-pointer items-center gap-2.5 rounded-control px-3 font-medium outline-none transition-colors duration-150 ease-out data-[highlighted]:bg-hover"
+            >
+              <span class="flex size-[18px] items-center justify-center">
+                <DropdownMenuItemIndicator>
+                  <Check :size="18" aria-hidden="true" />
+                </DropdownMenuItemIndicator>
+              </span>
+              <span>{{ language.name }}</span>
+            </DropdownMenuRadioItem>
+          </DropdownMenuRadioGroup>
+        </template>
         <DropdownMenuSeparator class="my-1 h-px bg-border" />
         <DropdownMenuItem
           data-action="logout"