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/app/i18n.ts

@@ -17,6 +17,11 @@ const locale = ref(typeof document === 'undefined' ? 'pl' : document.documentEle
 /** The locale the loaded bundle resolved to (meta.locale of GET /lang). */
 export const currentLocale = readonly(locale)
 
+const locales = ref<string[]>([])
+
+/** The supported admin locales (meta.locales of GET /lang). */
+export const availableLocales = readonly(locales)
+
 export function setBundle(next: LangBundle, nextLocale?: string): void {
   bundle.value = next
   if (nextLocale) {
@@ -27,12 +32,17 @@ export function setBundle(next: LangBundle, nextLocale?: string): void {
   }
 }
 
-/** Loads the backend::lang bundle for the admin's locale. Public endpoint. */
-export async function loadStrings(): Promise<void> {
-  const { data, response } = await api.GET('/lang')
+/**
+ * Loads the backend::lang bundle. Public endpoint. preferred (the admin's
+ * chosen language) is sent as ?locale=; the server uses it only when it is
+ * a supported admin locale, else the request locale.
+ */
+export async function loadStrings(preferred?: string): Promise<void> {
+  const { data, response } = await api.GET('/lang', preferred ? { params: { query: { locale: preferred } } } : {})
   if (!response.ok || !data) {
     return
   }
+  locales.value = Array.isArray(data.meta.locales) ? [...data.meta.locales] : []
   setBundle(data.data, data.meta.locale)
 }