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/components/providers/intl-provider.tsx

@@ -0,0 +1,24 @@
+'use client';
+
+import { NextIntlClientProvider } from 'next-intl';
+import type { AbstractIntlMessages } from 'next-intl';
+import type { ReactNode } from 'react';
+import type { AppLocale } from '@/i18n/config';
+
+type Props = {
+  locale: AppLocale;
+  messages: AbstractIntlMessages;
+  children: ReactNode;
+};
+
+export function AppIntlProvider({ locale, messages, children }: Props) {
+  return (
+    <NextIntlClientProvider
+      locale={locale}
+      messages={messages}
+      timeZone='Asia/Shanghai'
+    >
+      {children}
+    </NextIntlClientProvider>
+  );
+}