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/App.vue

@@ -1,19 +1,11 @@
 <template>
   <v-app>
-      <NavBar />
-      <v-main>
-          <router-view />
-      </v-main>
+    <router-view />
   </v-app>
 </template>
 
 <script>
- import NavBar from './components/NavBar.vue';
-
  export default {
    name: 'App',
-   components: {
-     NavBar,
-   },
  }
 </script>