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/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>