feat(frontend): add next-intl bilingual i18n for core paths

Wire next-intl without locale routes, add zh-CN/en catalogs, language
switcher, and migrate layout/auth/settings UI copy. Document i18n rules
in AGENTS.md and keep static export builds working.

frontend/i18n/request.ts

@@ -0,0 +1,44 @@
+import { cookies, headers } from 'next/headers';
+import { getRequestConfig } from 'next-intl/server';
+import {
+  defaultLocale,
+  isAppLocale,
+  localeCookieName,
+  normalizeLocale,
+  pickLocaleFromAcceptLanguage,
+  type AppLocale,
+} from './config';
+
+async function resolveRequestLocale(): Promise<AppLocale> {
+  try {
+    const cookieStore = await cookies();
+    const cookieLocale = cookieStore.get(localeCookieName)?.value;
+    if (isAppLocale(cookieLocale)) {
+      return cookieLocale;
+    }
+    if (cookieLocale) {
+      return normalizeLocale(cookieLocale);
+    }
+  } catch {
+    // Static export / non-request contexts cannot read cookies.
+  }
+
+  try {
+    const headerStore = await headers();
+    const acceptLanguage = headerStore.get('accept-language');
+    return pickLocaleFromAcceptLanguage(acceptLanguage);
+  } catch {
+    // Static export build time.
+  }
+
+  return defaultLocale;
+}
+
+export default getRequestConfig(async () => {
+  const locale = await resolveRequestLocale();
+  const messages = (await import(`../messages/${locale}.json`)).default;
+  return {
+    locale,
+    messages,
+  };
+});