fix(frontend): harden auth routing and callback error handling

src/App.vue

@@ -1,6 +1,6 @@
 <template>
   <div class="min-h-screen bg-gray-50">
-    <nav v-if="!isLoginPage" class="bg-white border-b border-gray-200 px-6 py-3 flex items-center justify-between shadow-sm">
+    <nav v-if="showNav" class="bg-white border-b border-gray-200 px-6 py-3 flex items-center justify-between shadow-sm">
       <div class="flex items-center gap-6">
         <span class="font-bold text-gray-800 text-base">member.ose.tw</span>
         <router-link
@@ -56,7 +56,10 @@ const route = useRoute()
 const router = useRouter()
 const authStore = useAuthStore()
 
-const isLoginPage = computed(() => route.name === 'login')
+const showNav = computed(() => {
+  const onAuthPage = route.name === 'login' || route.name === 'auth-callback'
+  return authStore.isLoggedIn && !onAuthPage
+})
 
 function handleAdminNav(command) {
   const routes = {