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/AIOnboarding.tsx

@@ -360,7 +360,7 @@ export default function AIOnboarding({ onCancel, onComplete, categories, invento
         <div className="flex-1 flex flex-col gap-6 min-h-0 overflow-hidden">
           <div className="relative flex-1 min-h-0 rounded-[2.5rem] overflow-hidden border-4 border-slate-900 shadow-2xl bg-surface">
             <img src={image || undefined} className="w-full h-full object-contain" alt="Captured label" />
-            <div className="absolute inset-0 bg-gradient-to-t from-black/60 via-transparent to-transparent pointer-events-none" />
+            <div className="absolute inset-0 bg-black/30 pointer-events-none" />
             
             {uploading && (
               <div className="absolute inset-0 bg-background/60 flex flex-col items-center justify-center gap-4 z-10 transition-all">