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/shared/i18n/locale.ts

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+/**
+ * 项目 i18n 配置（without i18n routing 模式）。
+ *
+ * 设计决策（v3 引入完整 i18n 体系）：
+ * - 采用 next-intl 4.x，官方推荐的 Next.js App Router 方案
+ * - 不使用 `[locale]` 路由段，避免破坏现有 (auth)/(dashboard)/(onboarding) 路由组结构
+ * - locale 通过 cookie 持久化，SSR 时从 cookie 读取
+ * - 字典放在 shared/i18n/messages/，符合三层架构约束
+ *
+ * 支持的 locale：
+ * - zh-CN（默认）：简体中文
+ * - en：英文
+ */
+
+export const LOCALES = ["zh-CN", "en"] as const;
+export type Locale = (typeof LOCALES)[number];
+
+export const DEFAULT_LOCALE: Locale = "zh-CN";
+
+/** Cookie 名称，与 proxy.ts / request.ts 保持一致 */
+export const LOCALE_COOKIE = "NEXT_LOCALE";
+
+/** Cookie 属性：1 年有效期，全站可读 */
+export const LOCALE_COOKIE_OPTIONS = {
+  maxAge: 60 * 60 * 24 * 365,
+  path: "/",
+  sameSite: "lax" as const,
+};
+
+/**
+ * 校验字符串是否为受支持的 locale。
+ * 用于从 cookie/header 读取后的容错处理。
+ */
+export function isLocale(value: string | null | undefined): value is Locale {
+  return typeof value === "string" && (LOCALES as readonly string[]).includes(value);
+}
+
+/**
+ * 将任意字符串归一化为合法 locale，非法值回退到默认。
+ */
+export function normalizeLocale(value: string | null | undefined): Locale {
+  return isLocale(value) ? value : DEFAULT_LOCALE;
+}