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/router/index.tsx

@@ -1,5 +1,6 @@
 import { createBrowserRouter, Navigate, Outlet } from 'react-router-dom'
 import { useAuthStore } from '../stores/auth'
+import Layout from '../components/Layout'
 import LoginPage from '../pages/LoginPage'
 import RegisterPage from '../pages/RegisterPage'
 import ProjectPage from '../pages/ProjectPage'
@@ -25,20 +26,25 @@ export const router = createBrowserRouter([
     element: <ProtectedRoute />,
     children: [
       {
-        path: '/',
-        element: <Navigate to="/projects/proj-default" replace />,
-      },
-      {
-        path: '/projects/:projectId',
-        element: <ProjectPage />,
-      },
-      {
-        path: '/projects/:projectId/generate',
-        element: <GeneratePage />,
-      },
-      {
-        path: '/projects/:projectId/tasks/:taskId',
-        element: <ResultPage />,
+        element: <Layout />,
+        children: [
+          {
+            path: '/',
+            element: <Navigate to="/projects/proj-default" replace />,
+          },
+          {
+            path: '/projects/:projectId',
+            element: <ProjectPage />,
+          },
+          {
+            path: '/projects/:projectId/generate',
+            element: <GeneratePage />,
+          },
+          {
+            path: '/projects/:projectId/tasks/:taskId',
+            element: <ResultPage />,
+          },
+        ],
       },
     ],
   },