fix(web): solid background for the marketing mobile menu

The dropdown was bg/95 + backdrop-blur — fragile across mobile browsers
where backdrop-filter is unreliable, leaving 5% transparency that read as
"no background". Switch to a solid elevated panel with a soft shadow and
an explicit z-index.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/components/marketing-mobile-menu.tsx

@@ -27,7 +27,7 @@ export function MarketingMobileMenu() {
         {open ? <X size={18} /> : <Menu size={18} />}
       </button>
       {open && (
-        <div className="absolute inset-x-0 top-12 border-b border-[--color-border] bg-[--color-bg]/95 backdrop-blur-md">
+        <div className="absolute inset-x-0 top-12 z-40 border-b border-[--color-border] bg-[--color-bg-elevated] shadow-lg shadow-black/40">
           <nav className="mx-auto flex max-w-6xl flex-col px-6">
             {LINKS.map((l) => (
               <Link