fix(admin): 后台列表切换保留当前内容，去掉加载占位

用户档案、用户列表、内容筛选与公告页等数据到达后再换视图，避免滚动条闪动。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/app/admin/announcements/page.tsx

@@ -2,6 +2,7 @@ import type { Metadata } from "next";
 import { cookies } from "next/headers";
 import AnnouncementAdmin from "./AnnouncementAdmin";
 import { authCookieHeader } from "@/lib/cookies";
+import { fetchAdminAnnouncements, type Announcement } from "@/lib/api";
 import { getMeCached } from "@/lib/serverData";
 import { isAdminOrAbove } from "@/lib/roles";
 import Forbidden from "../Forbidden";
@@ -17,5 +18,15 @@ export default async function AdminAnnouncementsPage() {
   if (!me.user || !isAdminOrAbove(me.user.role)) {
     return <Forbidden text="公告管理仅管理员及以上角色可操作。" />;
   }
-  return <AnnouncementAdmin />;
+
+  let initial: Announcement[] = [];
+  let initError = "";
+  try {
+    const res = await fetchAdminAnnouncements(cookie || undefined);
+    initial = res.announcements ?? [];
+  } catch (e) {
+    initError = e instanceof Error ? e.message : "加载公告失败";
+  }
+
+  return <AnnouncementAdmin initial={initial} initError={initError} />;
 }