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/u/[id]/page.tsx

@@ -121,7 +121,7 @@ export default async function UserPage({ params, searchParams }: UserPageProps)
 
   return (
     <div>
-      <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)" }}>{u.nickname || u.username}</span>
@@ -276,7 +276,7 @@ export default async function UserPage({ params, searchParams }: UserPageProps)
       {/* ═══ 三栏：左导航 / 中内容 / 右信息 ═══ */}
       <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">
         {/* 左栏：Tab 导航 */}
-        <aside className="hidden lg:flex w-56 xl:w-60 shrink-0 sticky top-24 flex-col gap-4">
+        <aside className="hidden lg:flex w-56 xl:w-60 shrink-0 sticky top-[var(--rail-top)] flex-col gap-4">
           <section className="j13-module !p-2">
             <p
               className="px-2.5 pt-1.5 pb-2 text-[11.5px] font-bold tracking-[0.08em] uppercase"
@@ -341,7 +341,7 @@ export default async function UserPage({ params, searchParams }: UserPageProps)
         </div>
 
         {/* 右栏：账号信息 + 积分 + 安全提示 */}
-        <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">
+        <aside 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">
           <section className="j13-module">
             <p className="j13-module-title mb-3">
               <Hash size={15} style={{ color: "var(--accent)" }} /> 账号信息