Fix hero section structure across all product pages

Changes:
- Section class: relative now first, 3-stop gradient (from-primary-800 via-primary-700 to-primary-900)
- Added py-16 lg:py-24 padding
- Added relative z-10 to content wrapper
- All 22 product pages standardized

Reference: armflex.astro hero section structure

src/pages/grilles.astro

@@ -301,7 +301,7 @@ import BaseLayout from '@/layouts/BaseLayout.astro';
     </section>
 
     <!-- Contact CTA -->
-    <section class="py-16 bg-gradient-to-br from-primary-700 to-primary-600 text-white  relative overflow-hidden">
+    <section class="relative bg-gradient-to-br from-primary-800 via-primary-700 to-primary-900 text-white py-16 lg:py-24 overflow-hidden">
       <div class="max-w-4xl mx-auto px-4 sm:px-6 lg:px-8 text-center">
         <h2 class="text-xl sm:text-2xl font-bold mb-6">สนใจสินค้าของเรา?</h2>
         <p class="text-lg text-white/80 mb-8 max-w-2xl mx-auto">