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/components/site-doc/SiteDocShell.tsx

@@ -8,11 +8,14 @@ export default function SiteDocShell({
   left,
   right,
   children,
+  breadcrumb,
   mobileExtras,
 }: {
   left?: ReactNode;
   right?: ReactNode;
   children: ReactNode;
+  /** 中栏卡片上方的位置指引；桌面绝对定位，不占栅格高度 */
+  breadcrumb?: ReactNode;
   /** 窄屏下沉到正文之后的侧栏副本 */
   mobileExtras?: ReactNode;
 }) {
@@ -20,16 +23,19 @@ export default function SiteDocShell({
     <>
       <div className="grid grid-cols-1 items-start lg:grid-cols-[224px_minmax(0,1fr)_280px] xl:grid-cols-[240px_minmax(0,1fr)_300px] gap-x-6 xl:gap-x-7">
         <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="站点导航"
         >
           {left}
         </aside>
 
-        <div className="min-w-0">{children}</div>
+        <div className="min-w-0 relative">
+          {breadcrumb}
+          {children}
+        </div>
 
         <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"
         >
           {right}