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/CommunityPanels.tsx

@@ -450,8 +450,11 @@ function CommunityPanelsBody({
 
 export function CommunityPanels(props: PanelsProps) {
   return (
-    <aside className="hidden w-[280px] shrink-0 lg:block xl:w-[300px]" aria-label="社区面板">
-      <div className="sticky top-24 space-y-4">
+    <aside
+      className="hidden w-[280px] shrink-0 lg:block xl:w-[300px]"
+      aria-label="社区面板"
+    >
+      <div className="space-y-4">
         <CommunityPanelsBody {...props} />
       </div>
     </aside>