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/SiteChrome.tsx

@@ -4,6 +4,7 @@ import { useEffect, useLayoutEffect, useRef, useState } from "react";
 import { usePathname } from "next/navigation";
 import Header from "@/components/Header";
 import Footer from "@/components/Footer";
+import BackToTop from "@/components/BackToTop";
 import type { User } from "@/lib/api";
 
 function pathIsAdmin(pathname: string | null) {
@@ -11,6 +12,17 @@ function pathIsAdmin(pathname: string | null) {
   return pathname === "/admin" || pathname.startsWith("/admin/");
 }
 
+function pathIsChat(pathname: string | null) {
+  if (!pathname) return false;
+  return pathname === "/chat" || pathname.startsWith("/chat/");
+}
+
+/** 帖子详情（非 edit）移动端有底栏，回顶需抬高 */
+function pathIsPostDetail(pathname: string | null) {
+  if (!pathname) return false;
+  return /^\/post\/[^/]+$/.test(pathname);
+}
+
 /**
  * 公开站 chrome 闸门：/admin 不渲染 Header/Footer/1440 main。
  * 首屏用 middleware 的 x-pathname 初值保证 hydration 一致；
@@ -63,6 +75,9 @@ export default function SiteChrome({
     return <>{children}</>;
   }
 
+  // 聊天页锁视口、主滚动在对话区，窗口回顶无意义
+  const showBackToTop = !pathIsChat(pathname);
+
   return (
     <>
       <Header
@@ -76,11 +91,14 @@ export default function SiteChrome({
       <main
         id="main"
         tabIndex={-1}
-        className="flex-1 w-full max-w-[1440px] mx-auto px-4 sm:px-6 py-7 sm:py-9 outline-none"
+        className="flex-1 w-full max-w-[1440px] mx-auto px-4 sm:px-6 py-7 sm:py-[var(--main-pad-y)] outline-none"
       >
         {children}
       </main>
       <Footer siteName={siteName} />
+      {showBackToTop ? (
+        <BackToTop liftForMobileBar={pathIsPostDetail(pathname)} />
+      ) : null}
     </>
   );
 }