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/SiteDocBreadcrumb.tsx

@@ -3,24 +3,29 @@ import { ChevronRight } from "lucide-react";
 
 export type Crumb = { href?: string; label: string };
 
-/** 公文页面包屑 */
+/** 位置指引：桌面叠在中栏卡片上方的内边距里，不拉低卡片、不对齐到左栏 */
 export default function SiteDocBreadcrumb({ items }: { items: Crumb[] }) {
   return (
-    <nav className="meta mb-5 flex items-center gap-1.5 flex-wrap text-[13px]" aria-label="面包屑">
+    <nav
+      className="meta mb-2 flex items-center gap-1 flex-wrap text-[12.5px] leading-none lg:absolute lg:bottom-full lg:left-0 lg:right-0 lg:mb-0 lg:pb-2 lg:pl-1"
+      aria-label="面包屑"
+    >
       {items.map((item, i) => {
         const last = i === items.length - 1;
+        const cls = last ? "truncate" : "shrink-0";
         return (
-          <span key={`${item.label}-${i}`} className="inline-flex items-center gap-1.5 min-w-0">
-            {i > 0 ? <ChevronRight size={13} className="shrink-0" /> : null}
-            {item.href && !last ? (
-              <Link href={item.href} className="hover:text-[var(--accent)] transition-colors shrink-0">
+          <span key={`${item.label}-${i}`} className="inline-flex items-center gap-1 min-w-0">
+            {i > 0 ? <ChevronRight size={12} className="shrink-0 opacity-70" aria-hidden /> : null}
+            {item.href ? (
+              <Link
+                href={item.href}
+                className={`hover:text-[var(--accent)] transition-colors ${cls}`}
+                style={last ? { color: "var(--accent)" } : undefined}
+              >
                 {item.label}
               </Link>
             ) : (
-              <span
-                className={last ? "truncate" : "shrink-0"}
-                style={last ? { color: "var(--accent)" } : undefined}
-              >
+              <span className={cls} style={last ? { color: "var(--accent)" } : undefined}>
                 {item.label}
               </span>
             )}