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/i18n/actions.ts

@@ -0,0 +1,32 @@
+"use server"
+
+import { cookies } from "next/headers"
+import { revalidatePath } from "next/cache"
+
+import {
+  LOCALE_COOKIE,
+  LOCALE_COOKIE_OPTIONS,
+  type Locale,
+  isLocale,
+} from "@/shared/i18n/locale"
+
+/**
+ * 切换语言：写入 cookie + 触发页面刷新。
+ *
+ * 设计说明：
+ * - 不使用 URL 路由段，避免破坏现有路由组结构
+ * - cookie 持久化用户偏好，SSR 时由 i18n/request.ts 读取
+ * - revalidatePath 确保所有页面用新 locale 重新渲染
+ */
+export async function setLocaleAction(locale: string): Promise<{ success: boolean }> {
+  if (!isLocale(locale)) {
+    return { success: false }
+  }
+
+  const cookieStore = await cookies()
+  cookieStore.set(LOCALE_COOKIE, locale as Locale, LOCALE_COOKIE_OPTIONS)
+
+  // 刷新所有页面，让 SSR 用新 locale 重新渲染
+  revalidatePath("/", "layout")
+  return { success: true }
+}

src/i18n/request.ts

@@ -0,0 +1,40 @@
+import { getRequestConfig } from "next-intl/server";
+import { cookies } from "next/headers";
+
+import { DEFAULT_LOCALE, LOCALE_COOKIE, isLocale } from "@/shared/i18n/locale";
+
+/**
+ * next-intl 请求配置（without i18n routing 模式）。
+ *
+ * locale 来源优先级：
+ * 1. cookie `NEXT_LOCALE`（用户主动切换时写入）
+ * 2. 默认值 `zh-CN`
+ *
+ * 不使用 Accept-Language 自动协商，避免 SSR 与客户端 hydration 不一致。
+ * 用户切换语言时通过 setLocaleAction 写入 cookie + router.refresh()。
+ */
+export default getRequestConfig(async () => {
+  const cookieStore = await cookies();
+  const cookieValue = cookieStore.get(LOCALE_COOKIE)?.value;
+  const locale = isLocale(cookieValue) ? cookieValue : DEFAULT_LOCALE;
+
+  // 按命名空间拆分加载，避免单文件过大
+  const [common, auth, onboarding, classes, errors] = await Promise.all([
+    import(`@/shared/i18n/messages/${locale}/common.json`),
+    import(`@/shared/i18n/messages/${locale}/auth.json`),
+    import(`@/shared/i18n/messages/${locale}/onboarding.json`),
+    import(`@/shared/i18n/messages/${locale}/classes.json`),
+    import(`@/shared/i18n/messages/${locale}/errors.json`),
+  ]);
+
+  return {
+    locale,
+    messages: {
+      common: common.default,
+      auth: auth.default,
+      onboarding: onboarding.default,
+      classes: classes.default,
+      errors: errors.default,
+    },
+  };
+});