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/views/landing/LandingView.vue

@@ -6,10 +6,10 @@
     <!-- NAV -->
     <nav class="nav">
       <div class="container nav-inner">
-        <a href="/" class="brand">
+        <router-link to="/" class="brand">
           <span class="hex">⬢</span>
           <span>PURO AI</span>
-        </a>
+        </router-link>
         <div class="nav-links">
           <a href="#features">产品</a>
           <a href="/docs">文档</a>