fix(mobile-menu): make menu scrollable when content overflows

After adding the products dropdown (7 categories + 27 subcategory
links), the mobile menu was taller than typical phone screens
(~640px on iPhone SE). Without overflow handling, users on
short screens could not reach the bottom items (ผลงาน, ติดต่อเรา).

Added to mobile menu container:
- max-h-[calc(100vh-4rem)]: cap height at viewport minus header
- overflow-y-auto: enable internal scrollbar when content overflows
- overscroll-contain: prevent scroll chain to background page
  (iOS Safari specific)

Menu is now fully scrollable within its own container — page
content underneath stays still while user scrolls through menu
items.

src/layouts/BaseLayout.astro

@@ -418,7 +418,7 @@ const productLinks = [
     </nav>
 
     <!-- Mobile Menu -->
-    <div id="mobile-menu" class="lg:hidden hidden bg-white border-t border-neutral-100">
+    <div id="mobile-menu" class="lg:hidden hidden bg-white border-t border-neutral-100 max-h-[calc(100vh-4rem)] overflow-y-auto overscroll-contain">
       <div class="px-4 py-6 space-y-4">
         <a href="/" class="block text-neutral-600 hover:text-primary-600 font-medium py-2">หน้าแรก</a>
         <a href="/about-us" class="block text-neutral-600 hover:text-primary-600 font-medium py-2">เกี่ยวกับเรา</a>