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/admin/layout.tsx

@@ -28,7 +28,6 @@ export default async function AdminLayout({ children }: { children: React.ReactN
         id="main"
         tabIndex={-1}
         className="min-h-screen flex items-center justify-center px-4 outline-none"
-        style={{ background: "var(--bg)" }}
       >
         <div className="max-w-md w-full panel p-8 text-center">
           <span