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/post/[id]/edit/page.tsx

@@ -39,7 +39,7 @@ export default async function EditPostPage({ params }: PageProps) {
       data-compose-viewport
       className="j13-compose-page w-full"
     >
-      <nav className="meta mb-4 flex items-center gap-1.5 flex-wrap text-[13px] shrink-0">
+      <nav className="meta -mt-4 sm:-mt-6 mb-1.5 flex items-center gap-1 flex-wrap text-[12.5px] leading-none shrink-0">
         <Link href="/" className="hover:text-[var(--accent)] transition-colors">
           首页
         </Link>