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/services/http.js

@@ -1,5 +1,6 @@
 import axios from 'axios';
 import AuthService from '@/services/auth';
+import router from '@/router';
 
 const http = axios.create({
   baseURL: import.meta.env.VITE_DJANGO_BASE_URL,
@@ -32,7 +33,7 @@ http.interceptors.response.use(
         return http.request(originalRequest);
       } catch (refreshError) {
         AuthService.logout();
-        window.location.href = '/autenticarse';
+        router.push('/autenticarse');
         return Promise.reject(refreshError);
       }
     }