fix: resolve mobile layout issues and redesign catalog header

- Add 'app' prop to NavBar for proper Vuetify layout integration
- Fix mobile z-index: page-header now compensates for NavBar height (64px)
- Fix cart visibility: position cart above footer instead of hidden underneath
- Redesign header: white background with subtle shadow instead of blue gradient
- Expand catalog layout: increase catalog width (md=10 lg=9) for better content space
- Optimize mobile search: expand search field to full width, hide title on mobile
- Adjust mobile padding: increase to 100px to account for footer + collapsed cart

src/components/NavBar.vue

@@ -1,5 +1,5 @@
 <template>
-    <v-app-bar color="primary" prominent>
+    <v-app-bar color="primary" prominent app>
     <v-app-bar-nav-icon variant="text" @click.stop="drawer = !drawer"></v-app-bar-nav-icon>
     <v-toolbar-title>Menu</v-toolbar-title>
     <v-spacer></v-spacer>