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/announcement/[id]/page.tsx

@@ -103,6 +103,15 @@ export default async function AnnouncementPage({ params }: PageProps) {
       <SiteDocShell
         left={left}
         right={right}
+        breadcrumb={
+          <SiteDocBreadcrumb
+            items={[
+              { href: "/", label: "首页" },
+              { href: "/announcements", label: "站点公告" },
+              { label: a.title },
+            ]}
+          />
+        }
         mobileExtras={
           <>
             {toc.length > 0 ? <PostToc items={toc} /> : null}
@@ -111,14 +120,6 @@ export default async function AnnouncementPage({ params }: PageProps) {
           </>
         }
       >
-        <SiteDocBreadcrumb
-          items={[
-            { href: "/", label: "首页" },
-            { href: "/announcements", label: "站点公告" },
-            { label: a.title },
-          ]}
-        />
-
         <article className="panel p-6 sm:p-8">
           <SiteDocMasthead
             kicker="站点公告"