fix: unify chinese language code as zh-CN

.unipi/docs/specs/2026-05-25-frontend-backend-api-requirements.md

@@ -15,7 +15,7 @@ status: draft
 - API base：前端通过 `VITE_API_URL` 指向后端；本地可同源 `/api`。
 - 上传文件可通过 `/uploads/...` 或完整 URL 返回；前端会用 `assetUrl()` 处理相对路径。
 - 所有时间字段使用 ISO 8601 字符串，例如 `2026-05-24T14:42:00.000Z`。
-- 语言字段：`zh` / `en` / `ja` / `ko` / `vi` / `id` / `ms`；默认语言为 `en`。中文只有简体 `zh`，没有繁体中文。
+- 语言字段：`zh-CN` / `en` / `ja` / `ko` / `vi` / `id` / `ms`；默认语言为 `en`。中文只有简体 `zh-CN`，没有繁体中文。
 - 错误格式：非 2xx + text/plain 或 JSON 均可；前端会显示错误文本。
 - Admin 接口需要 `Authorization: Bearer <token>`。
 
@@ -57,7 +57,7 @@ type Post = {
   id: string;
   categoryId: number;
   categorySlug: string;
-  language: "zh" | "en" | "ja" | "ko" | "vi" | "id" | "ms";
+  language: "zh-CN" | "en" | "ja" | "ko" | "vi" | "id" | "ms";
   text?: string;
   attachments: Attachment[];
   isRecommended: boolean;
@@ -449,7 +449,7 @@ Request:
 ```ts
 type UpsertPostPayload = {
   categoryId: number;
-  language: "zh" | "en" | "ja" | "ko" | "vi" | "id" | "ms";
+  language: "zh-CN" | "en" | "ja" | "ko" | "vi" | "id" | "ms";
   text?: string;
   attachments: Attachment[];
   isPublic: boolean;