feat(router): mount Landing at / with auth-aware redirect

- / (anonymous) → LandingView
- / (authenticated) → redirects to /dashboard via new meta.redirectIfAuth
- Remove temporary /landing-preview route (Task 2 helper)
- RouteMeta TS augmentation for redirectIfAuth
- LandingView brand link uses router-link (was <a href>, causing SPA reload)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/router/index.ts

@@ -36,15 +36,6 @@ const routes: RouteRecordRaw[] = [
       title: 'Home'
     }
   },
-  {
-    path: '/landing-preview',
-    name: 'LandingPreview',
-    component: () => import('@/views/landing/LandingView.vue'),
-    meta: {
-      requiresAuth: false,
-      title: 'PURO AI'
-    }
-  },
   {
     path: '/login',
     name: 'Login',
@@ -133,7 +124,13 @@ const routes: RouteRecordRaw[] = [
   // ==================== User Routes ====================
   {
     path: '/',
-    redirect: '/home'
+    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: '/dashboard',
@@ -542,6 +539,13 @@ router.beforeEach((to, _from, next) => {
     authInitialized = true
   }
 
+  // Auth-aware redirect for public pages that should bounce authenticated users elsewhere
+  // (e.g., Landing / at `/` redirects to /dashboard if user is logged in)
+  const redirectIfAuth = to.meta.redirectIfAuth
+  if (redirectIfAuth && authStore.isAuthenticated) {
+    return next(redirectIfAuth)
+  }
+
   // Set page title
   const appStore = useAppStore()
   // For custom pages, use menu item label as document title