feat: SAC 0.7.3 custom error pages 401/403/404/429

Centered logo pages with event explanation and allowed actions; router
and apiFetch redirect to them on auth, permission, not-found, and rate-limit errors.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/App.vue

@@ -15,7 +15,8 @@ import AppSidebar from "./components/AppSidebar.vue";
 import { refreshSessionRole } from "./router";
 
 const route = useRoute();
-const showShell = computed(() => route.path !== "/login" && !!getToken());
+const publicLayout = computed(() => route.path === "/login" || route.meta.standalone === true);
+const showShell = computed(() => !publicLayout.value && !!getToken());
 
 onMounted(() => {
   if (getToken()) {