refactor: remove decorative gradients per distill principles

- Scanner: Replace gradient scan line with solid primary color (keeps animation, removes glow)
- AIOnboarding: Replace gradient overlay with solid semi-transparent black
- IdentityCheckOverlay: Remove purely decorative 'accent light' gradient

Aligns with design principle: no decoration without function. Industrial aesthetic demands honest materials and intentional visual treatments.

frontend/components/IdentityCheckOverlay.tsx

@@ -62,8 +62,6 @@ const IdentityCheckOverlay = memo(({ show, users, onAuthenticated }: IdentityChe
   return (
     <div className="fixed inset-0 z-[100] flex items-center justify-center p-4 bg-background/90 animate-in fade-in duration-500">
       <div className="bg-surface border border-slate-800 rounded-[2.5rem] p-8 sm:p-10 max-w-sm w-full shadow-2xl space-y-8 animate-in zoom-in-95 duration-300 relative overflow-hidden">
-        {/* Subtle accent light */}
-        <div className="absolute top-0 left-1/2 -translate-x-1/2 w-40 h-1 bg-gradient-to-r from-transparent via-primary/50 to-transparent blur-sm" />
         
         <div className="text-center space-y-3">
           <div className="w-20 h-20 bg-primary/10 text-primary rounded-[2rem] flex items-center justify-center mx-auto mb-6 border border-primary/20 shadow-xl shadow-primary/5">