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/home/WorkspaceNav.tsx

@@ -16,8 +16,12 @@ export function WorkspaceNav({ boards }: { boards: BoardWithCount[]; user?: User
   const activeBoard = Number(params.get("board")) || 0;
 
   return (
-    <nav className="hidden lg:block w-56 xl:w-60 shrink-0" aria-label="工作台导航">
-      <div className="sticky top-24 space-y-5">
+    // sticky 必须挂在 flex 子项上：父级 items-start 会把内层缩成内容高，失去粘滞行程
+    <nav
+      className="hidden lg:block w-56 xl:w-60 shrink-0 sticky top-[var(--rail-top)] max-h-[var(--rail-max-h)] j13-rail-scroll"
+      aria-label="工作台导航"
+    >
+      <div className="space-y-5">
         {/* 「全部讨论」是重置筛选的动作入口，与板块目录分区展示；卡片样式与下方板块一致 */}
         <div className="space-y-0.5">
           <Link href="/" className="j13-bcard" aria-current={activeBoard === 0 ? "page" : undefined}>