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/components/Purchase.vue

@@ -13,10 +13,15 @@
 
       <!-- Loading -->
       <template v-if="loading">
-        <v-skeleton-loader
-          class="mb-4 rounded-lg"
-          type="card-heading, list-item-two-line, list-item-two-line"
-        ></v-skeleton-loader>
+        <v-sheet class="d-flex flex-column align-center justify-center pa-12 rounded-lg" elevation="2">
+          <v-progress-circular
+            indeterminate
+            color="primary"
+            size="64"
+            width="6"
+          ></v-progress-circular>
+          <p class="text-body-1 text-grey mt-4">Cargando datos...</p>
+        </v-sheet>
       </template>
 
       <template v-else>
@@ -246,7 +251,7 @@
             <v-row align="center">
               <v-col cols="12" md="6">
                 <v-select
-                  :items="payment_methods"
+                  :items="payment_methods || []"
                   v-model="purchase.payment_method"
                   item-title="text"
                   item-value="value"
@@ -337,7 +342,7 @@
        show_alert_purchase: false,
        client_search: '',
        product_search: '',
-       payment_methods: null,
+       payment_methods: [],
        purchase: {
          date: this.getCurrentDate(),
          customer: null,
@@ -471,15 +476,13 @@
      formatPrice(price) {
        return new Intl.NumberFormat('es-CO', { style: 'currency', currency: 'COP', minimumFractionDigits: 0 }).format(price);
      },
-   },
-     mounted() {
-         this.fetchClients();
-     }
- };
+    }
+  };
 </script>
 <style>
 .page-header {
-  background: linear-gradient(135deg, #1565C0 0%, #0D47A1 100%);
+  background: linear-gradient(135deg, #1565C0 0%, #0D47A1 100%) !important;
+  color: white !important;
 }
 
 .product-line:nth-child(odd) {