feat(layout): 认证页面统一导航栏，含主题切换与退出按钮

新增 Layout 组件作为认证路由的公共布局，提供 sticky 导航栏，
移除各页面重复的顶栏代码。

frontend/src/pages/GeneratePage.tsx

@@ -1,5 +1,5 @@
 import { useEffect } from 'react'
-import { Link, useNavigate, useParams } from 'react-router-dom'
+import { useNavigate, useParams } from 'react-router-dom'
 import { useTaskStore } from '../stores/task'
 import { useGenerationStore } from '../stores/generation'
 import GenerateForm from '../components/GenerateForm'
@@ -46,28 +46,7 @@ export default function GeneratePage() {
 
   return (
     <div className="container" style={{ paddingTop: 24, paddingBottom: 40 }}>
-      {/* 顶栏 */}
-      <div
-        style={{
-          display: 'flex',
-          justifyContent: 'space-between',
-          alignItems: 'center',
-          marginBottom: 32,
-        }}
-      >
-        <h1 style={{ fontSize: 24 }}>新建生成</h1>
-        <Link
-          to={`/projects/${projectId}`}
-          className="btn-secondary"
-          style={{
-            padding: '8px 20px',
-            fontSize: 13,
-            borderRadius: 'var(--radius)',
-          }}
-        >
-          返回工程
-        </Link>
-      </div>
+      <h1 style={{ fontSize: 24, marginBottom: 32 }}>新建生成</h1>
 
       {/* 生成表单 */}
       {status === 'idle' || status === 'submitting' ? (