refactor(portal): extract PortalLayout so Nav/Footer persist across routes

将 Landing/Docs/Pricing 的 Nav + Footer 提取到共享 PortalLayout。
Router 改为嵌套结构，路由切换时 router-view 内容变化，Nav 本身
不重挂载，消除切页时的 UI 抖动（真·SPA 行为）。

- new: components/layout/PortalLayout.vue（Nav + router-view + Footer）
- router: /、/docs、/pricing 作为 PortalLayout 的子路由
- i18n: 新增 portal.nav.* 命名空间；删除重复的 docs.nav.* / pricing.nav.* / landing.nav.*
- router: scrollBehavior 支持 hash 锚点跳转（offset 80px 绕开 sticky nav）
- router-link 使用 active-class/exact-active-class prop 替代硬编码 class="active"

frontend/src/router/index.ts

@@ -120,32 +120,38 @@ const routes: RouteRecordRaw[] = [
       title: 'Key Usage',
     }
   },
-  {
-    path: '/docs',
-    name: 'Docs',
-    component: () => import('@/views/docs/DocsView.vue'),
-    meta: {
-      requiresAuth: false,
-      title: 'PURO AI · 文档'
-    }
-  },
-  {
-    path: '/pricing',
-    name: 'pricing',
-    component: () => import('@/views/pricing/PricingView.vue'),
-    meta: { requiresAuth: false, title: 'Pricing · PURO AI' }
-  },
 
-  // ==================== User Routes ====================
+  // ==================== Portal Routes (shared PortalLayout) ====================
   {
     path: '/',
-    name: 'Landing',
-    component: () => import('@/views/landing/LandingView.vue'),
-    meta: {
-      requiresAuth: false,
-      title: 'PURO AI — 你的 AI 订阅，已经付过钱了',
-      redirectIfAuth: '/dashboard'
-    }
+    component: () => import('@/components/layout/PortalLayout.vue'),
+    children: [
+      {
+        path: '',
+        name: 'Landing',
+        component: () => import('@/views/landing/LandingView.vue'),
+        meta: {
+          requiresAuth: false,
+          title: 'PURO AI — 你的 AI 订阅，已经付过钱了',
+          redirectIfAuth: '/dashboard'
+        }
+      },
+      {
+        path: 'docs',
+        name: 'Docs',
+        component: () => import('@/views/docs/DocsView.vue'),
+        meta: {
+          requiresAuth: false,
+          title: 'PURO AI · 文档'
+        }
+      },
+      {
+        path: 'pricing',
+        name: 'pricing',
+        component: () => import('@/views/pricing/PricingView.vue'),
+        meta: { requiresAuth: false, title: 'Pricing · PURO AI' }
+      },
+    ]
   },
   {
     path: '/dashboard',
@@ -521,11 +527,15 @@ const routes: RouteRecordRaw[] = [
 const router = createRouter({
   history: createWebHistory(import.meta.env.BASE_URL),
   routes,
-  scrollBehavior(_to, _from, savedPosition) {
+  scrollBehavior(to, _from, savedPosition) {
     // Scroll to saved position when using browser back/forward
     if (savedPosition) {
       return savedPosition
     }
+    // Scroll to hash target (anchor link) — offset by sticky nav height
+    if (to.hash) {
+      return { el: to.hash, behavior: 'smooth', top: 80 }
+    }
     // Scroll to top for new routes
     return { top: 0 }
   }