feat(ui): 全站画布氛围层、侧栏 sticky 对齐与长页回顶

用顶栏/主栏高度令牌消除 sticky 首滚空走，面包屑叠到中栏上方，公开页增加回顶浮钮。

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

frontend/app/page.tsx

@@ -17,6 +17,7 @@ import PostRow from "@/components/PostRow";
 import FeedTabs, { type FeedSort } from "@/components/home/FeedTabs";
 import FeedNewTip from "@/components/home/FeedNewTip";
 import { WorkspaceNav, WorkspaceNavMobile } from "@/components/home/WorkspaceNav";
+import { SiteDocBreadcrumb } from "@/components/site-doc";
 import { CommunityPanels, CommunityPanelsMobile } from "@/components/home/CommunityPanels";
 import {
   BoardPanels,
@@ -166,11 +167,21 @@ export default async function HomePage({ searchParams }: HomePageProps) {
     <div>
       <WorkspaceNavMobile boards={boards} user={user} />
 
-      <div className="flex items-start gap-6 xl:gap-7 mt-5 lg:mt-0">
+      <div className={`flex items-start gap-6 xl:gap-7 ${activeBoard ? "" : "mt-5 lg:mt-0"}`}>
         <WorkspaceNav boards={boards} user={user} />
 
         {/* 中栏：tab 栏与帖子流为同一个整体卡片（左栏切板块只替换这一列） */}
-        <div className="flex-1 min-w-0">
+        <div className="flex-1 min-w-0 relative">
+          {activeBoard ? (
+            <div className="mt-4 lg:mt-0">
+              <SiteDocBreadcrumb
+                items={[
+                  { href: "/", label: "首页" },
+                  { label: activeBoard.name },
+                ]}
+              />
+            </div>
+          ) : null}
           <h1 className="sr-only">{heading}</h1>
 
           <div className="panel overflow-hidden">