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/MobilePostBar.tsx

@@ -1,13 +1,12 @@
 "use client";
 
-import { useEffect, useState } from "react";
-import { ArrowUp, MessageSquare } from "lucide-react";
+import { MessageSquare } from "lucide-react";
 import LikeButton from "./LikeButton";
 import { scrollBehavior } from "@/lib/motion";
 
 /**
  * 帖子详情移动端浮动操作条（lg 以下显示）：
- * 长文读到中途也能点赞 / 跳评论框 / 回顶部，不必滚到文末。
+ * 长文读到中途也能点赞 / 跳评论框；回顶由全局 BackToTop 承担。
  */
 export default function MobilePostBar({
   postId,
@@ -20,15 +19,6 @@ export default function MobilePostBar({
   likeCount: number;
   allowComments?: boolean;
 }) {
-  const [showTop, setShowTop] = useState(false);
-
-  useEffect(() => {
-    const onScroll = () => setShowTop(window.scrollY > 400);
-    onScroll();
-    window.addEventListener("scroll", onScroll, { passive: true });
-    return () => window.removeEventListener("scroll", onScroll);
-  }, []);
-
   const scrollToComments = () => {
     document.getElementById("comment-input")?.scrollIntoView({
       behavior: scrollBehavior(),
@@ -36,13 +26,6 @@ export default function MobilePostBar({
     });
   };
 
-  const scrollToTop = () => {
-    window.scrollTo({
-      top: 0,
-      behavior: scrollBehavior(),
-    });
-  };
-
   return (
     <div className="lg:hidden fixed bottom-4 left-1/2 -translate-x-1/2 z-40" role="navigation" aria-label="阅读操作">
       <div
@@ -66,21 +49,9 @@ export default function MobilePostBar({
             <MessageSquare size={18} />
           </button>
         ) : null}
-        {/* 放大点赞按钮的触控区域（样式见 globals.css .j13-bar-like） */}
         <div className="j13-bar-like">
           <LikeButton postId={postId} liked={liked} likeCount={likeCount} variant="compact" />
         </div>
-        {showTop && (
-          <button
-            type="button"
-            onClick={scrollToTop}
-            className="w-11 h-11 flex items-center justify-center rounded-full transition-colors hover:bg-[var(--accent-soft)] focus-visible:bg-[var(--accent-soft)] j13-toast-in"
-            style={{ color: "var(--ink-2)" }}
-            aria-label="回到顶部"
-          >
-            <ArrowUp size={18} />
-          </button>
-        )}
       </div>
     </div>
   );