fix: resolve white screen crashes and restructure layout hierarchy

- Move NavBar from App.vue to layouts/default.vue to fix nested v-app/v-main
- Replace VSkeletonLoader with v-progress-circular to avoid genStructure crash
- Initialize payment_methods as [] and add fallback in v-select
- Remove duplicate fetchClients call from mounted()
- Add authStore.user check in admin route guard
- Replace window.location.href with router.push for SPA navigation
- Add !important to page-header gradient styles

src/router/index.js

@@ -35,7 +35,7 @@ router.beforeEach((to, from, next) => {
 
   if (requiresAuth && !isAuthenticated) {
     next({ path: '/autenticarse', replace: true })
-  } else if (requiresAdmin && !authStore.isAdmin) {
+  } else if (requiresAdmin && !authStore.isAdmin && authStore.user) {
     next({ path: '/', replace: true })
   } else {
     next()