feat(02-06): auth layout switching + admin role guard (GAPs 2, 3, 4)

- App.vue: conditionally renders AuthLayout for auth routes, app shell otherwise
- router/index.js: meta.layout='auth' on all four auth routes
- router/index.js: meta.requiresAdmin=true on /admin route
- router/index.js: beforeEach role check redirects non-admin to /
- router/index.js: /account redirects to /settings

frontend/src/App.vue

@@ -1,5 +1,6 @@
 <template>
-  <div class="flex h-screen overflow-hidden">
+  <AuthLayout v-if="route.meta.layout === 'auth'" />
+  <div v-else class="flex h-screen overflow-hidden">
     <AppSidebar />
     <main class="flex-1 overflow-y-auto">
       <router-view />
@@ -8,10 +9,13 @@
 </template>
 
 <script setup>
-import AppSidebar from './components/layout/AppSidebar.vue'
-import { useTopicsStore } from './stores/topics.js'
 import { onMounted } from 'vue'
+import { useRoute } from 'vue-router'
+import AppSidebar from './components/layout/AppSidebar.vue'
+import AuthLayout from './layouts/AuthLayout.vue'
+import { useTopicsStore } from './stores/topics.js'
 
+const route = useRoute()
 const topicsStore = useTopicsStore()
 onMounted(() => topicsStore.fetchTopics())
 </script>