feat: 知识库一键后台保存 + 对话置顶仅首页 + 排行榜页面增强

- 知识库「保存」改为一键触发，无需填表；后端经 BackgroundTasks 异步沉淀到 wiki，点击立即返回 {queued:true}
- 对话列表置顶分区仅在第一页展示，翻页后不再重复顶置
- 公开/管理员排行榜页面重构与维度增强；排行榜页改为整页统一滚动

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend-web/src/pages/AdminUserLeaderboardPage.tsx

@@ -1229,6 +1229,8 @@ export default function AdminUserLeaderboardPage() {
   return (
     <div className="flex size-full min-h-0 bg-[#FFF7F7] text-[#231815] dark:bg-[#011226] dark:text-white">
       <main className="flex min-w-0 flex-1 flex-col">
+        {/* 整页统一滚动：标题与操作工具条不再固定置顶，随内容一起滚动，避免可视区域被压缩 */}
+        <ScrollArea className="min-h-0 flex-1">
         <div className="border-b border-[#F1D6D6] bg-white px-6 py-5 dark:border-[#213D5E] dark:bg-[#02244C]">
           <div className="flex flex-col gap-4">
             {/* 标题维度：仅承载页面标题与说明 */}
@@ -1448,7 +1450,6 @@ export default function AdminUserLeaderboardPage() {
           </div>
         </div>
 
-        <ScrollArea className="min-h-0 flex-1">
           <div className="space-y-5 p-6">
             {isLoading && (
               <div className="flex flex-col items-center justify-center gap-3 py-20 text-center text-sm text-[#5F6368] dark:text-slate-400">

frontend-web/src/pages/ChatsPage.tsx

@@ -76,8 +76,9 @@ export default function ChatsPage() {
     });
   }, [threads, search, onlyFavorites]);
 
-  // Pinned conversations are surfaced in their own section and excluded from
-  // the paginated list so they stay visible regardless of the current page.
+  // Pinned conversations are surfaced in their own section on the first page
+  // only, and excluded from the paginated list (so they don't repeat on every
+  // page as the user pages through history).
   const pinnedThreads = useMemo(
     () => filteredThreads.filter(isThreadPinned),
     [filteredThreads],
@@ -200,7 +201,7 @@ export default function ChatsPage() {
           <main className="min-h-0 flex-1">
             <ScrollArea className="size-full py-4">
               <div className="mx-auto flex size-full max-w-(--container-width-md) flex-col">
-                {pinnedThreads.length > 0 && (
+                {currentPage === 1 && pinnedThreads.length > 0 && (
                   <>
                     <div className="text-muted-foreground/70 px-4 pb-1 pt-2 text-xs font-semibold tracking-wider uppercase">
                       {t.chats.pinnedSection}