feat(admin-i18n): add cookie-based locale runtime and switcher baseline

apps/admin/src/components/admin-locale-switcher.tsx

@@ -0,0 +1,41 @@
+"use client"
+
+import { type AppLocale, localeLabels, locales } from "@cms/i18n"
+import { useRouter } from "next/navigation"
+import { useTransition } from "react"
+
+import { ADMIN_LOCALE_COOKIE } from "@/i18n/shared"
+import { useAdminI18n, useAdminT } from "@/providers/admin-i18n-provider"
+
+export function AdminLocaleSwitcher() {
+  const router = useRouter()
+  const [isPending, startTransition] = useTransition()
+  const { locale } = useAdminI18n()
+  const t = useAdminT()
+
+  return (
+    <label className="inline-flex items-center gap-2 text-sm text-neutral-700">
+      <span>{t("common.language", "Language")}</span>
+      <select
+        className="rounded-md border border-neutral-300 bg-white px-2 py-1 text-sm"
+        value={locale}
+        disabled={isPending}
+        onChange={(event) => {
+          const nextLocale = event.target.value as AppLocale
+          // biome-ignore lint/suspicious/noDocumentCookie: locale preference is intentionally persisted client-side.
+          document.cookie = `${ADMIN_LOCALE_COOKIE}=${nextLocale}; Path=/; Max-Age=31536000; SameSite=Lax`
+
+          startTransition(() => {
+            router.refresh()
+          })
+        }}
+      >
+        {locales.map((value) => (
+          <option key={value} value={value}>
+            {t(`common.localeNames.${value}`, localeLabels[value])} ({localeLabels[value]})
+          </option>
+        ))}
+      </select>
+    </label>
+  )
+}