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.

src/app/(onboarding)/onboarding/page.tsx

@@ -0,0 +1,70 @@
+import { Suspense } from "react"
+import { redirect } from "next/navigation"
+import type { Metadata } from "next"
+import { getTranslations } from "next-intl/server"
+
+import { auth } from "@/auth"
+import { getOnboardingStatus } from "@/modules/onboarding/data-access"
+import { OnboardingStepper } from "@/modules/onboarding/components/onboarding-stepper"
+
+export async function generateMetadata(): Promise<Metadata> {
+  const t = await getTranslations("onboarding")
+  return {
+    title: t("title"),
+    description: t("description"),
+  }
+}
+
+export default async function OnboardingPage() {
+  const session = await auth()
+  const userId = session?.user?.id
+
+  if (!userId) {
+    redirect("/login")
+  }
+
+  // 已完成 onboarding 的用户不应停留在此页
+  if (session.user.onboarded) {
+    redirect("/dashboard")
+  }
+
+  const status = await getOnboardingStatus(userId)
+
+  // 二次校验：DB 层面已 onboarded 但 session 未刷新
+  if (!status.required) {
+    redirect("/dashboard")
+  }
+
+  return (
+    <main className="flex min-h-screen items-center justify-center bg-muted/30 p-4">
+      <div className="w-full max-w-2xl rounded-lg border bg-background p-8 shadow-sm">
+        {/* useSearchParams 需要 Suspense 边界（P1-1 URL query 持久化步骤） */}
+        <Suspense fallback={<OnboardingLoading />}>
+          <OnboardingStepper initialStatus={status} />
+        </Suspense>
+      </div>
+    </main>
+  )
+}
+
+function OnboardingLoading() {
+  return (
+    <div className="grid gap-6" aria-busy="true" aria-live="polite">
+      <div className="grid gap-1.5">
+        <div className="h-7 w-40 animate-pulse rounded bg-muted" />
+        <div className="h-4 w-64 animate-pulse rounded bg-muted" />
+      </div>
+      <div className="flex items-center gap-2">
+        <div className="h-1 flex-1 rounded bg-muted" />
+        <div className="h-1 flex-1 rounded bg-muted" />
+        <div className="h-1 flex-1 rounded bg-muted" />
+        <div className="h-1 flex-1 rounded bg-muted" />
+      </div>
+      <div className="grid gap-4">
+        <div className="h-10 animate-pulse rounded bg-muted" />
+        <div className="h-10 animate-pulse rounded bg-muted" />
+        <div className="h-10 animate-pulse rounded bg-muted" />
+      </div>
+    </div>
+  )
+}