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/AnnouncementAdmin.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { useCallback, useEffect, useState } from "react";
+import { useCallback, useState } from "react";
 import {
   Plus,
   Pencil,
@@ -51,32 +51,30 @@ const EMPTY_FORM: FormState = {
   published: true,
 };
 
-export default function AnnouncementAdmin() {
-  const [list, setList] = useState<Announcement[]>([]);
-  const [loading, setLoading] = useState(true);
-  const [loadError, setLoadError] = useState("");
+export default function AnnouncementAdmin({
+  initial,
+  initError,
+}: {
+  initial: Announcement[];
+  initError: string;
+}) {
+  const [list, setList] = useState(initial);
+  const [loadError, setLoadError] = useState(initError);
   const [form, setForm] = useState<FormState | null>(null);
   const [saving, setSaving] = useState(false);
   const [deletingId, setDeletingId] = useState<number | null>(null);
   const [deleteTarget, setDeleteTarget] = useState<Announcement | null>(null);
 
   const load = useCallback(async () => {
-    setLoading(true);
     try {
       const res = await apiAdminListAnnouncements();
       setList(res.announcements);
       setLoadError("");
     } catch (e) {
       setLoadError(e instanceof Error ? e.message : "加载公告失败");
-    } finally {
-      setLoading(false);
     }
   }, []);
 
-  useEffect(() => {
-    load();
-  }, [load]);
-
   const startCreate = () => setForm({ ...EMPTY_FORM });
   const startEdit = (a: Announcement) =>
     setForm({
@@ -263,11 +261,7 @@ export default function AnnouncementAdmin() {
             <span className="meta text-[12px] tabular-nums">共 {list.length} 条</span>
           </div>
 
-          {loading ? (
-            <div className="flex items-center justify-center gap-2 py-16 meta text-[13px]">
-              <Loader2 size={16} className="animate-spin" /> 加载中…
-            </div>
-          ) : loadError ? (
+          {loadError ? (
             <p className="alert-error m-4">{loadError}</p>
           ) : list.length === 0 ? (
             <AdminEmpty