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>
   );
 }

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>

frontend/components/home/WorkspaceNav.tsx

@@ -16,8 +16,12 @@ export function WorkspaceNav({ boards }: { boards: BoardWithCount[]; user?: User
   const activeBoard = Number(params.get("board")) || 0;
 
   return (
-    <nav className="hidden lg:block w-56 xl:w-60 shrink-0" aria-label="工作台导航">
-      <div className="sticky top-24 space-y-5">
+    // sticky 必须挂在 flex 子项上：父级 items-start 会把内层缩成内容高，失去粘滞行程
+    <nav
+      className="hidden lg:block w-56 xl:w-60 shrink-0 sticky top-[var(--rail-top)] max-h-[var(--rail-max-h)] j13-rail-scroll"
+      aria-label="工作台导航"
+    >
+      <div className="space-y-5">
         {/* 「全部讨论」是重置筛选的动作入口，与板块目录分区展示；卡片样式与下方板块一致 */}
         <div className="space-y-0.5">
           <Link href="/" className="j13-bcard" aria-current={activeBoard === 0 ? "page" : undefined}>