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/BoardPanels.tsx

@@ -283,8 +283,11 @@ function BoardPanelsBody(props: BoardPanelsProps) {
 
 export function BoardPanels(props: BoardPanelsProps) {
   return (
-    <aside className="hidden w-[280px] shrink-0 lg:block xl:w-[300px]" aria-label="板块面板">
-      <div className="sticky top-24 space-y-4">
+    <aside
+      className="hidden w-[280px] shrink-0 lg:block xl:w-[300px]"
+      aria-label="板块面板"
+    >
+      <div className="space-y-4">
         <BoardPanelsBody {...props} />
       </div>
     </aside>
@@ -302,18 +305,19 @@ export function BoardPanelsMobile(props: BoardPanelsProps) {
 /** 板块不存在时的简卡 */
 export function BoardMissingPanel() {
   return (
-    <aside className="hidden w-[280px] shrink-0 lg:block xl:w-[300px]" aria-label="板块面板">
-      <div className="sticky top-24">
-        <section className="j13-module">
-          <p className="j13-module-title mb-2">
-            <LayoutGrid size={15} style={{ color: "var(--ink-3)" }} /> 板块
-          </p>
-          <p className="meta">板块不存在或已删除</p>
-          <Link href="/" className="btn btn-line btn-sm mt-3 w-full justify-center">
-            返回全部讨论
-          </Link>
-        </section>
-      </div>
+    <aside
+      className="hidden w-[280px] shrink-0 lg:block xl:w-[300px]"
+      aria-label="板块面板"
+    >
+      <section className="j13-module">
+        <p className="j13-module-title mb-2">
+          <LayoutGrid size={15} style={{ color: "var(--ink-3)" }} /> 板块
+        </p>
+        <p className="meta">板块不存在或已删除</p>
+        <Link href="/" className="btn btn-line btn-sm mt-3 w-full justify-center">
+          返回全部讨论
+        </Link>
+      </section>
     </aside>
   );
 }