feat(web): 后台主题切换与移动端溢出菜单，设置保存按钮上移 topbar

- 后台 topbar 右侧常驻主题切换按钮（复用博客端三态循环逻辑），
  tooltip 新增 tip-below 朝下变体适配近视口顶部的 topbar
- <768px 侧栏隐藏时主题/返回博客/退出登录收进「···」溢出菜单；
  博客 <640px 首页/标签收进 header 溢出菜单（带 active 高亮）
- 新增通用 MoreMenu 组件：点击外部/Escape/选中项自动关闭，
  面板用 --popover 令牌 + 0.15s 淡入，与现有卡片风格一致
- 站点设置保存按钮移至 topbar 标题右侧，删除底部 sticky 保存条
- 修复两处回归：tip-below 气泡居中锚定越界产生页面横向滚动条
  （改右对齐）；pn-site-menu-btn 基础 display 规则源码顺序在
  640px MQ 之后导致移动端菜单按钮不显示（前移至博客 header 区）
- 已用 Playwright 在 1440×900 / 390×844 实测后台四页与博客
  菜单展开/跳转均无溢出

web/src/components/Layout.tsx

@@ -1,20 +1,72 @@
+import { useEffect, useRef, useState } from 'react'
 import { Link, NavLink, Outlet, useLocation, useNavigate } from 'react-router'
-import { Moon, Sun, Monitor, ExternalLink, House, PenLine, Tags, Trash2, Settings, LogOut } from 'lucide-react'
+import {
+  Moon, Sun, Monitor, ExternalLink, House, PenLine, MoreHorizontal, Tags, Trash2, Settings, LogOut,
+} from 'lucide-react'
 import { useSiteTitle } from '../features/site'
 import { useTheme } from '../lib/theme'
 import { useAuth } from '../lib/auth'
 
+/* ── 溢出菜单：触发按钮 + 下拉面板（点击外部 / Esc / 选中项关闭） ── */
+
+function MoreMenu({ triggerClassName, triggerLabel, children }: {
+  triggerClassName: string
+  triggerLabel: string
+  children: React.ReactNode
+}) {
+  const [open, setOpen] = useState(false)
+  const ref = useRef<HTMLDivElement>(null)
+  useEffect(() => {
+    if (!open) return
+    const onPointer = (e: PointerEvent) => {
+      if (ref.current && !ref.current.contains(e.target as Node)) setOpen(false)
+    }
+    const onKey = (e: KeyboardEvent) => {
+      if (e.key === 'Escape') setOpen(false)
+    }
+    document.addEventListener('pointerdown', onPointer)
+    document.addEventListener('keydown', onKey)
+    return () => {
+      document.removeEventListener('pointerdown', onPointer)
+      document.removeEventListener('keydown', onKey)
+    }
+  }, [open])
+  return (
+    <div className="pn-moremenu" ref={ref}>
+      <button
+        type="button"
+        className={triggerClassName}
+        aria-haspopup="menu"
+        aria-expanded={open}
+        aria-label={triggerLabel}
+        onClick={() => setOpen((v) => !v)}
+      >
+        <MoreHorizontal size={15} />
+      </button>
+      {open && (
+        <div className="pn-moremenu-panel" role="menu" onClick={() => setOpen(false)}>
+          {children}
+        </div>
+      )}
+    </div>
+  )
+}
+
 /* ── 博客端：居中单栏 720px 布局（博客系列设计稿） ─────────── */
 
-function ThemeButton() {
+function ThemeButton({ className = 'pn-site-navlink', tip = 'native' }: {
+  className?: string
+  tip?: 'native' | 'css'
+}) {
   const { mode, setMode } = useTheme()
   const next = mode === 'light' ? 'dark' : mode === 'dark' ? 'system' : 'light'
   const title = `主题：${mode === 'light' ? '浅色' : mode === 'dark' ? '深色' : '跟随系统'}（点击切换）`
   return (
     <button
       type="button"
-      className="pn-site-navlink"
+      className={className}
-      title={title}
+      title={tip === 'native' ? title : undefined}
+      data-tip={tip === 'css' ? title : undefined}
       aria-label={title}
       onClick={() => setMode(next)}
     >
@@ -42,6 +94,15 @@ export function BlogLayout() {
               <ExternalLink size={14} />管理
             </NavLink>
             <ThemeButton />
+            {/* <640px 首页/标签收进溢出菜单 */}
+            <MoreMenu triggerClassName="pn-site-navlink pn-site-menu-btn" triggerLabel="更多导航">
+              <NavLink to="/" end className={({ isActive }) => `pn-moremenu-item${isActive ? ' active' : ''}`}>
+                <House size={15} />首页
+              </NavLink>
+              <NavLink to="/tags" className={({ isActive }) => `pn-moremenu-item${isActive ? ' active' : ''}`}>
+                <Tags size={15} />标签
+              </NavLink>
+            </MoreMenu>
           </nav>
         </div>
       </header>
@@ -75,10 +136,35 @@ export function BlogLayout() {
 /* ── 管理端：sidebar + topbar + 移动端 tabbar（dashboard 设计稿） ── */
 
 export function AdminTopbar({ title, children }: { title: string; children?: React.ReactNode }) {
+  const { logout } = useAuth()
+  const navigate = useNavigate()
+  const { mode, setMode } = useTheme()
+  const next = mode === 'light' ? 'dark' : mode === 'dark' ? 'system' : 'light'
+  const modeLabel = mode === 'light' ? '浅色' : mode === 'dark' ? '深色' : '跟随系统'
   return (
     <header className="pn-admin-topbar">
       <h1>{title}</h1>
-      {children && <div className="pn-admin-actions">{children}</div>}
+      <div className="pn-admin-actions">
+        {children}
+        <ThemeButton className="pn-admin-iconbtn tip-below pn-admin-desktop-act" tip="css" />
+        {/* <768px 侧栏隐藏、topbar 空间有限：主题/返回博客/退出登录收进溢出菜单 */}
+        <MoreMenu triggerClassName="pn-admin-iconbtn pn-admin-mobile-act" triggerLabel="更多操作">
+          <button type="button" className="pn-moremenu-item" onClick={() => setMode(next)}>
+            {mode === 'light' ? <Sun size={15} /> : mode === 'dark' ? <Moon size={15} /> : <Monitor size={15} />}
+            主题：{modeLabel}
+          </button>
+          <a href="/" target="_blank" rel="noreferrer" className="pn-moremenu-item">
+            <ExternalLink size={15} />返回博客
+          </a>
+          <button
+            type="button"
+            className="pn-moremenu-item"
+            onClick={() => void logout().then(() => navigate('/'))}
+          >
+            <LogOut size={15} />退出登录
+          </button>
+        </MoreMenu>
+      </div>
     </header>
   )
 }

web/src/index.css

@@ -489,6 +489,11 @@ button:focus-visible {
   width: 14px;
   height: 14px;
 }
+/* 博客端溢出菜单触发按钮：<640px 显示（基础规则须在 640px MQ 之前，
+   否则同特异性下源码顺序靠后者覆盖 MQ 内的 display:inline-flex） */
+.pn-site-menu-btn {
+  display: none;
+}
 .pn-site-main {
   flex: 1;
   width: 100%;
@@ -547,6 +552,9 @@ button:focus-visible {
   .pn-site-navlink.hide-sm {
     display: none;
   }
+  .pn-site-menu-btn {
+    display: inline-flex;
+  }
 }
 
 /* ============================================================
@@ -1658,6 +1666,99 @@ button:focus-visible {
 .pn-admin-iconbtn:focus-visible::before {
   opacity: 1;
 }
+/* tooltip 朝下变体（topbar 贴近视口顶部，气泡须向下弹出）。
+   气泡右对齐按钮：居中锚定会越过 topbar 右缘产生页面横向滚动条 */
+.pn-admin-iconbtn.tip-below::after {
+  bottom: auto;
+  top: calc(100% + 10px);
+  left: auto;
+  right: 0;
+  transform: translate(0, -2px);
+}
+.pn-admin-iconbtn.tip-below::before {
+  bottom: auto;
+  top: 100%;
+  border-top-color: transparent;
+  border-bottom-color: var(--primary);
+}
+.pn-admin-iconbtn.tip-below:hover::after,
+.pn-admin-iconbtn.tip-below:focus-visible::after {
+  transform: translate(0, 0);
+}
+/* 侧栏隐藏时的移动端兜底按钮（<768px 显示） */
+.pn-admin-mobile-act {
+  display: none;
+}
+/* 溢出菜单：面板与菜单项（博客 header / 后台 topbar 共用） */
+.pn-moremenu {
+  position: relative;
+}
+.pn-moremenu-panel {
+  position: absolute;
+  top: calc(100% + 8px);
+  right: 0;
+  z-index: 60;
+  min-width: 150px;
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+  padding: 6px;
+  border: 1px solid var(--border);
+  border-radius: var(--radius-md);
+  background: var(--popover);
+  box-shadow: var(--shadow-sm);
+  animation: pn-moremenu-in 0.15s ease;
+}
+@keyframes pn-moremenu-in {
+  from {
+    opacity: 0;
+    transform: translateY(-4px);
+  }
+  to {
+    opacity: 1;
+    transform: none;
+  }
+}
+.pn-moremenu-item {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+  padding: 8px 10px;
+  border: 0;
+  border-radius: var(--radius-sm);
+  background: transparent;
+  color: var(--foreground);
+  font-size: 13px;
+  font-weight: 500;
+  line-height: 1.4;
+  text-align: left;
+  text-decoration: none;
+  white-space: nowrap;
+  cursor: pointer;
+  transition: background-color 0.15s ease;
+}
+.pn-moremenu-item:hover,
+.pn-moremenu-item.active {
+  background: var(--muted);
+  color: var(--foreground);
+}
+.pn-moremenu-item.active {
+  font-weight: 600;
+}
+.pn-moremenu-item svg {
+  width: 15px;
+  height: 15px;
+  flex: none;
+  color: var(--muted-foreground);
+}
+.pn-moremenu-item.active svg {
+  color: var(--foreground);
+}
+/* 无 data-tip 的图标按钮不渲染空 tooltip 气泡（如菜单触发按钮） */
+.pn-admin-iconbtn:not([data-tip])::after,
+.pn-admin-iconbtn:not([data-tip])::before {
+  display: none;
+}
 .pn-admin-avatar {
   width: 28px;
   height: 28px;
@@ -1799,6 +1900,23 @@ button:focus-visible {
   }
   .pn-admin-topbar h1 {
     font-size: 16px;
+    min-width: 0;
+    overflow: hidden;
+    text-overflow: ellipsis;
+    white-space: nowrap;
+  }
+  .pn-admin-actions {
+    gap: 6px;
+  }
+  .pn-admin-actions .btn {
+    padding-left: 10px;
+    padding-right: 10px;
+  }
+  .pn-admin-mobile-act {
+    display: inline-flex;
+  }
+  .pn-admin-desktop-act {
+    display: none;
   }
   .pn-admin-search {
     display: none;
@@ -2588,41 +2706,10 @@ textarea.pn-as-input {
   padding-top: 16px;
   border-top: 1px solid var(--border);
 }
-.pn-as-save-bar {
-  position: sticky;
-  bottom: 20px;
-  display: flex;
-  align-items: center;
-  justify-content: space-between;
-  gap: 16px;
-  padding: 12px 16px;
-  background: var(--card);
-  border: 1px solid var(--border);
-  border-radius: var(--radius-md);
-  box-shadow: var(--shadow-sm);
-}
-.pn-as-save-hint {
-  font-size: 12.5px;
-  color: var(--muted-foreground);
-}
-.pn-as-save-bar .btn {
-  min-height: 36px;
-  padding: 0 16px;
-  font-size: 13.5px;
-  flex: none;
-}
-.pn-as-save-bar .btn svg {
-  width: 15px;
-  height: 15px;
-}
 @media (max-width: 767px) {
   .pn-as-card {
     padding: 20px;
   }
-  .pn-as-save-bar {
-    bottom: 88px;
-    flex-wrap: wrap;
-  }
 }
 
 /* ============================================================

web/src/pages/AdminSettings.tsx

@@ -47,7 +47,17 @@ export default function AdminSettings() {
 
   return (
     <>
-      <AdminTopbar title="站点设置" />
+      <AdminTopbar title="站点设置">
+        <button
+          type="button"
+          className="btn primary"
+          disabled={saveSettings.isPending}
+          onClick={() => saveSettings.mutate()}
+        >
+          <Save size={15} />
+          {saveSettings.isPending ? '保存中…' : '保存更改'}
+        </button>
+      </AdminTopbar>
 
       <div className="pn-admin-content pn-as-content">
         <section className="card pn-as-card">
@@ -104,19 +114,6 @@ export default function AdminSettings() {
         </section>
 
         <ChangePassword onToast={setToast} />
-
-        <div className="pn-as-save-bar">
-          <span className="pn-as-save-hint">更改将在保存后应用到公开站点</span>
-          <button
-            type="button"
-            className="btn primary"
-            disabled={saveSettings.isPending}
-            onClick={() => saveSettings.mutate()}
-          >
-            <Save size={15} />
-            {saveSettings.isPending ? '保存中…' : '保存更改'}
-          </button>
-        </div>
       </div>
 
       <Toast message={toast?.msg ?? null} kind={toast?.kind} onDone={() => setToast(null)} />