feat(landing): Use central button tailwind variant

landing/app/app.vue

@@ -61,11 +61,8 @@
             base: 'text-gray-600 dark:text-gray-300 hover:text-primary-600 dark:hover:text-primary-400'
           }"
         />
-        <UButton
-          to="/#newsletter"
-          class="hidden sm:flex btn-gradient px-5 py-2.5 rounded-xl font-semibold shadow-lg shadow-primary-500/25 hover:shadow-primary-500/40 transition-shadow"
-        >
-          <span>{{ $t('common.stayInformed') }}</span>
+        <UButton to="/#newsletter" variant="gradient" class="hidden sm:flex px-5 py-2.5">
+          {{ $t('common.stayInformed') }}
         </UButton>
       </template>
 
@@ -79,7 +76,7 @@
           }"
         />
         <div class="mt-6 pt-6 border-t border-gray-200 dark:border-gray-800">
-          <UButton to="#newsletter" size="lg" block class="btn-gradient rounded-xl font-semibold">
+          <UButton to="#newsletter" size="lg" block variant="gradient">
             {{ $t('common.stayInformed') }}
           </UButton>
         </div>