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">

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">

frontend/components/Scanner.tsx

@@ -225,7 +225,7 @@ export default function Scanner({ onScanSuccess, onOCRMatch, paused }: ScannerPr
             <div className="absolute bottom-0 right-0 w-10 h-10 border-b-4 border-r-4 border-primary rounded-br-2xl shadow-[0_0_15px_rgba(59,130,246,0.5)]" />
             
             {isStarted && !paused && !isSelecting && (
-                 <div className="absolute top-0 left-0 right-0 h-1 bg-gradient-to-r from-transparent via-primary to-transparent shadow-[0_0_20px_rgba(59,130,246,0.8)] animate-scan-fast" />
+                 <div className="absolute top-0 left-0 right-0 h-1 bg-primary animate-scan-fast" />
             )}
           </div>
         </div>