feat: introduce i18n system and class invitation codes

Add complete i18n infrastructure using next-intl (cookie-driven, without i18n routing) with zh-CN/en dictionary files, locale switcher, and NextIntlClientProvider in root layout. Add class invitation code system with new class_invitation_codes table, data-access layer (generate/validate/consume/revoke), server actions with permission checks, rate limiting, and audit logging. Add class-invitation-manager UI component. Refactor onboarding stepper to use i18n translations and accept new invitation code format (6-char alphanumeric) with backward compatibility for legacy 6-digit codes.

next.config.ts

@@ -1,7 +1,10 @@
 import type { NextConfig } from "next";
+import createNextIntlPlugin from "next-intl/plugin";
+
+const withNextIntl = createNextIntlPlugin("./src/i18n/request.ts");
 
 const nextConfig: NextConfig = {
   output: "standalone",
 };
 
-export default nextConfig;
+export default withNextIntl(nextConfig);