fix: unify chinese language code as zh-CN

src/components/messageStream/MessageStream.tsx

@@ -26,7 +26,7 @@ export function MessageStream({ scope }: MessageStreamProps) {
   const { items, isLoading, error, hasMore, loadMore, reset } =
     usePostStream(params);
   const groups = useGroupedByDay(items, lang);
-  const retryLabel = lang === "zh" ? "重试" : "Retry";
+  const retryLabel = lang === "zh-CN" ? "重试" : "Retry";
 
   const sentinelRef = useRef<HTMLDivElement>(null);
   const hasMoreRef = useRef(hasMore);

src/components/messageStream/hooks/useGroupedByDay.test.ts

@@ -8,7 +8,7 @@ function makePost(id: string, isoDate: string): Post {
     id,
     categoryId: 1,
     categorySlug: "x",
-    language: "zh",
+    language: "zh-CN",
     attachments: [],
     isRecommended: false,
     publishedAt: isoDate,
@@ -25,7 +25,7 @@ describe("useGroupedByDay", () => {
       makePost("c", "2026-02-28T01:00:00.000Z"),
       makePost("d", "2026-05-16T12:00:00.000Z"),
     ];
-    const { result } = renderHook(() => useGroupedByDay(posts, "zh"));
+    const { result } = renderHook(() => useGroupedByDay(posts, "zh-CN"));
     expect(result.current.length).toBeGreaterThanOrEqual(2);
     const allIds = result.current.flatMap((g) => g.items.map((p) => p.id));
     expect(allIds).toEqual(["a", "b", "c", "d"]);
@@ -47,7 +47,7 @@ describe("useGroupedByDay", () => {
   });
 
   it("returns empty array for empty input", () => {
-    const { result } = renderHook(() => useGroupedByDay([], "zh"));
+    const { result } = renderHook(() => useGroupedByDay([], "zh-CN"));
     expect(result.current).toEqual([]);
   });
 });

src/components/messageStream/hooks/useGroupedByDay.ts

@@ -35,10 +35,10 @@ function dayLabel(iso: string, lang: string): string {
     a.getMonth() === b.getMonth() &&
     a.getDate() === b.getDate();
   if (isSameDay(d, today)) {
-    return lang === "zh" ? "今天" : "Today";
+    return lang === "zh-CN" ? "今天" : "Today";
   }
   if (isSameDay(d, yesterday)) {
-    return lang === "zh" ? "昨天" : "Yesterday";
+    return lang === "zh-CN" ? "昨天" : "Yesterday";
   }
   return new Intl.DateTimeFormat(localeFor(lang), {
     month: "long",