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/notifications/page.tsx

@@ -32,7 +32,7 @@ export default async function NotificationsPage({ searchParams }: NotificationsP
 
   return (
     <div className="max-w-2xl mx-auto">
-      <nav className="meta mb-5 flex items-center gap-1.5 text-[13px]">
+      <nav className="meta -mt-4 sm:-mt-6 mb-1.5 flex items-center gap-1 text-[12.5px] leading-none">
         <Link href="/" className="hover:text-[var(--accent)] transition-colors">首页</Link>
         <ChevronRight size={13} />
         <span style={{ color: "var(--accent)" }}>通知中心</span>