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/layouts/default.vue

@@ -1,13 +1,14 @@
 <template>
-  <v-app>
+  <div>
+    <NavBar />
     <v-main>
       <router-view />
     </v-main>
-
     <AppFooter />
-  </v-app>
+  </div>
 </template>
 
 <script setup>
-  //
+  import NavBar from '@/components/NavBar.vue';
+  import AppFooter from '@/components/AppFooter.vue';
 </script>