feat(admin): 管理后台布局改为视口固定，Header和侧边栏固定，内容区独立滚动

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

offer_frontend/src/layouts/AdminLayout.vue

@@ -1,6 +1,6 @@
 <template>
-  <el-container style="min-height: 100vh;">
-    <el-header style="background:#001529;display:flex;align-items:center;justify-content:space-between;border-bottom:1px solid #333;">
+  <el-container style="height:100vh;overflow:hidden;flex-direction:column;">
+    <el-header style="background:#001529;display:flex;align-items:center;justify-content:space-between;border-bottom:1px solid #333;flex-shrink:0;">
       <div style="color:#fff;font-weight:bold;font-size:16px;">管理后台</div>
       <el-dropdown>
         <div style="color:#fff;cursor:pointer;display:flex;align-items:center;gap:8px;">
@@ -14,8 +14,8 @@
         </template>
       </el-dropdown>
     </el-header>
-    <el-container>
-      <el-aside width="200px" style="background:#001529;">
+    <el-container style="flex:1;overflow:hidden;">
+      <el-aside width="200px" style="background:#001529;overflow-y:auto;height:100%;">
         <div style="padding:20px;color:#fff;font-weight:bold;font-size:16px;">菜单</div>
         <el-menu router :default-active="$route.path" background-color="#001529" text-color="#fff" active-text-color="#409eff">
           <el-menu-item index="/admin/jobs">职位管理</el-menu-item>
@@ -26,7 +26,7 @@
           </template>
         </el-menu>
       </el-aside>
-      <el-main><router-view /></el-main>
+      <el-main style="overflow-y:auto;height:100%;"><router-view /></el-main>
     </el-container>
   </el-container>
 </template>