feat: 访客模式极简界面，隐藏所有导航和登录入口

- 访客：顶部标题栏 + 深色模式图标，无侧边栏导航
- 访客：只能查看机器列表、搜索、筛选，无任何操作按钮
- 访客：无登录提示，无登录入口
- 管理员：完整侧边栏 + 操作按钮
- 登录页需手动访问 /login

web/src/components/MainLayout.vue

@@ -1,6 +1,7 @@
 <template>
   <div class="layout">
-    <aside class="sidebar">
+    <!-- 管理员侧边栏 -->
+    <aside class="sidebar" v-if="isAdmin">
       <div class="brand">
         <div class="brand-logo">
           <img src="/logo.svg" alt="LAN Manager" style="width:22px;height:22px;" />
@@ -12,33 +13,22 @@
       </div>
 
       <nav class="nav">
-        <!-- 管理员：全部导航 -->
-        <template v-if="isAdmin">
-          <router-link to="/dashboard" class="nav-item" active-class="active">
-            <el-icon><Odometer /></el-icon>
-            <span>仪表盘</span>
-          </router-link>
-          <router-link to="/machines" class="nav-item" active-class="active">
-            <el-icon><Monitor /></el-icon>
-            <span>机器列表</span>
-          </router-link>
-          <router-link to="/topology" class="nav-item" active-class="active">
-            <el-icon><Share /></el-icon>
-            <span>拓扑图</span>
-          </router-link>
-          <router-link to="/logs" class="nav-item" active-class="active">
-            <el-icon><Document /></el-icon>
-            <span>操作日志</span>
-          </router-link>
-        </template>
-
-        <!-- 访客：只能看到机器列表 -->
-        <template v-else>
-          <router-link to="/machines" class="nav-item" active-class="active">
-            <el-icon><Monitor /></el-icon>
-            <span>机器列表</span>
-          </router-link>
-        </template>
+        <router-link to="/dashboard" class="nav-item" active-class="active">
+          <el-icon><Odometer /></el-icon>
+          <span>仪表盘</span>
+        </router-link>
+        <router-link to="/machines" class="nav-item" active-class="active">
+          <el-icon><Monitor /></el-icon>
+          <span>机器列表</span>
+        </router-link>
+        <router-link to="/topology" class="nav-item" active-class="active">
+          <el-icon><Share /></el-icon>
+          <span>拓扑图</span>
+        </router-link>
+        <router-link to="/logs" class="nav-item" active-class="active">
+          <el-icon><Document /></el-icon>
+          <span>操作日志</span>
+        </router-link>
       </nav>
 
       <div class="sidebar-footer">
@@ -54,7 +44,6 @@
           </div>
         </div>
         <div class="footer-actions" v-if="isLoggedIn">
-          <!-- 管理员才有修改密码 -->
           <el-button v-if="isAdmin" text class="footer-btn" @click="openChangePassword">
             <el-icon><Lock /></el-icon>
             <span>修改密码</span>
@@ -67,7 +56,18 @@
       </div>
     </aside>
 
-    <main class="main">
+    <!-- 访客：顶部极简标题栏 -->
+    <header class="guest-header" v-if="!isAdmin">
+      <div class="guest-brand">
+        <img src="/logo.svg" alt="LAN Manager" style="width:22px;height:22px;" />
+        <span>LAN Manager</span>
+      </div>
+      <div class="guest-theme-toggle" @click="toggleTheme">
+        <el-icon><component :is="isDark ? Sunny : Moon" /></el-icon>
+      </div>
+    </header>
+
+    <main class="main" :class="{ 'guest-main': !isAdmin }">
       <div class="main-inner">
         <router-view />
       </div>
@@ -390,6 +390,50 @@ html.dark .brand-logo {
   padding: 10px;
 }
 
+.guest-main {
+  margin-left: 0;
+  padding-top: 48px;
+}
+
+.guest-header {
+  position: fixed;
+  top: 0;
+  left: 0;
+  right: 0;
+  height: 48px;
+  background: var(--sidebar-bg);
+  border-bottom: 1px solid var(--border);
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 0 16px;
+  z-index: 100;
+}
+.guest-brand {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  font-weight: 600;
+  font-size: 14px;
+  color: var(--text-primary);
+}
+.guest-theme-toggle {
+  width: 32px;
+  height: 32px;
+  border-radius: 50%;
+  background: var(--card-bg);
+  border: 1px solid var(--border);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  cursor: pointer;
+  color: var(--text);
+  transition: all .2s ease;
+}
+.guest-theme-toggle:hover {
+  background: var(--hover-bg);
+}
+
 @media (max-width: 768px) {
   .sidebar { width: 64px; }
   .brand-text, .nav-item span, .user-meta, .footer-btn span, .theme-toggle span { display: none; }
@@ -398,5 +442,6 @@ html.dark .brand-logo {
   .user-info { justify-content: center; padding: 10px; }
   .theme-toggle { justify-content: center; padding: 10px; }
   .main { margin-left: 64px; }
+  .guest-main { margin-left: 0; }
 }
 </style>

web/src/views/MachineList.vue

@@ -1,11 +1,5 @@
 <template>
   <div class="page">
-    <!-- 访客提示 -->
-    <div v-if="!isAdmin" class="guest-page-notice">
-      <el-icon><Warning /></el-icon>
-      <span>当前为只读模式</span>
-    </div>
-
     <!-- Toolbar -->
     <div class="toolbar">
       <div class="search-wrap">