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/announcements/page.tsx

@@ -132,15 +132,16 @@ export default async function AnnouncementsListPage({ searchParams }: PageProps)
     <SiteDocShell
       left={left}
       right={right}
+      breadcrumb={
+        <SiteDocBreadcrumb
+          items={[
+            { href: "/", label: "首页" },
+            { label: "站点公告" },
+          ]}
+        />
+      }
       mobileExtras={<SiteDocPageNav pages={sidebarPages} title="关于本站" />}
     >
-      <SiteDocBreadcrumb
-        items={[
-          { href: "/", label: "首页" },
-          { label: "站点公告" },
-        ]}
-      />
-
       <header className="mb-5">
         <h1
           className="text-[24px] sm:text-[28px] font-extrabold tracking-tight flex items-center gap-2"