feat(component): show admin menu only for admins

src/App.vue

@@ -49,7 +49,7 @@ async function handleLogout() {
           </svg>
           <span class="nav-text">搜索</span>
         </router-link>
-        <router-link v-if="auth.isLoggedIn" to="/admin" title="管理后台">
+        <router-link v-if="auth.isAdmin" to="/admin" title="管理后台">
           <svg class="nav-icon" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true">
             <rect x="3" y="3" width="7" height="7" />
             <rect x="14" y="3" width="7" height="7" />