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/post/[id]/page.tsx

@@ -30,6 +30,7 @@ import CommentSection from "@/components/CommentSection";
 import PostActions from "@/components/PostActions";
 import LikeButton from "@/components/LikeButton";
 import PostToc from "@/components/PostToc";
+import { SiteDocBreadcrumb } from "@/components/site-doc";
 import MobilePostBar from "@/components/MobilePostBar";
 import Avatar from "@/components/Avatar";
 import RoleBadge from "@/components/RoleBadge";
@@ -557,7 +558,7 @@ export default async function PostDetailPage({ params, searchParams }: PageProps
   return (
     <>
       <script type="application/ld+json" dangerouslySetInnerHTML={{ __html: JSON.stringify(jsonLd) }} />
-      {/* 移动端浮动操作条：长文阅读中途可点赞 / 跳评论 / 回顶部 */}
+      {/* 移动端浮动操作条：长文读到中途可点赞 / 跳评论框 */}
       <MobilePostBar
         postId={id}
         liked={post.liked}
@@ -571,7 +572,7 @@ export default async function PostDetailPage({ params, searchParams }: PageProps
         {/* sticky 必须直接挂在 grid 子项上：aside 作为 grid item，其可粘行程
             等于行高（正文高度）；内层包裹会被 items-start 缩成内容高而失去行程 */}
         <aside
-          className="hidden lg:flex w-56 xl:w-60 shrink-0 sticky top-24 flex-col gap-4 max-h-[calc(100vh_-_7rem)] overflow-hidden"
+          className="hidden lg:flex w-56 xl:w-60 shrink-0 sticky top-[var(--rail-top)] flex-col gap-4 max-h-[var(--rail-max-h)] overflow-hidden"
           aria-label="帖子导航"
         >
           <BoardCard board={post.board} />
@@ -580,16 +581,14 @@ export default async function PostDetailPage({ params, searchParams }: PageProps
         </aside>
 
         {/* 中栏：正文主体 */}
-        <div className="min-w-0">
+        <div className="min-w-0 relative">
+          <SiteDocBreadcrumb
+            items={[
+              { href: "/", label: "首页" },
+              { href: `/?board=${post.board.id}`, label: post.board.name },
+            ]}
+          />
           <article className="w-full">
-            <nav className="meta mb-5 flex items-center gap-1.5 flex-wrap text-[13px]">
-              <Link href="/" className="hover:text-[var(--accent)] transition-colors">首页</Link>
-              <ChevronRight size={13} />
-              <Link href={`/?board=${post.board.id}`} className="hover:text-[var(--accent)] transition-colors">
-                {post.board.name}
-              </Link>
-            </nav>
-
             <div className="panel p-6 sm:p-8">
               <header className="mb-7">
                 <div className="flex items-center gap-2 mb-4 flex-wrap">
@@ -810,7 +809,7 @@ export default async function PostDetailPage({ params, searchParams }: PageProps
 
         {/* 右栏：目录 / 同板块热议（阅读数据已并入标题下元信息条） */}
         <aside
-          className="hidden lg:flex w-[280px] xl:w-[300px] shrink-0 sticky top-24 flex-col gap-4 max-h-[calc(100vh_-_7rem)] overflow-hidden"
+          className="hidden lg:flex w-[280px] xl:w-[300px] shrink-0 sticky top-[var(--rail-top)] flex-col gap-4 max-h-[var(--rail-max-h)] overflow-hidden"
           aria-label="阅读辅助"
         >
           {toc.length > 0 && <PostToc items={toc} />}