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/next.config.ts

@@ -1,4 +1,7 @@
 import type { NextConfig } from 'next';
+import createNextIntlPlugin from 'next-intl/plugin';
+
+const withNextIntl = createNextIntlPlugin('./i18n/request.ts');
 
 const isExport = process.env.NEXT_STANDALONE_EXPORT === 'true';
 
@@ -32,4 +35,4 @@ const nextConfig: NextConfig = {
       }),
 };
 
-export default nextConfig;
+export default withNextIntl(nextConfig);