fix: resolve TypeScript build error in AIOnboarding and Scanner components

- Fix null type error in AIOnboarding.tsx line 352: img src attribute
- Fix similar issue in Scanner.tsx line 237: capturedImage null handling
- Use '|| undefined' pattern to satisfy React 19/Next.js 15 type requirements
- Both components now properly handle null/undefined image states

frontend/components/Scanner.tsx

@@ -234,7 +234,7 @@ export default function Scanner({ onScanSuccess, onOCRMatch, paused }: ScannerPr
         {isSelecting && capturedImage && (
           <div className="absolute inset-0 z-50 bg-slate-950 flex flex-col">
             <div className="relative flex-1 bg-black flex items-center justify-center overflow-hidden">
-               <img src={capturedImage} className="max-w-full max-h-full object-contain" id="ocr-canvas-preview" />
+               <img src={capturedImage || undefined} className="max-w-full max-h-full object-contain" id="ocr-canvas-preview" />
                <div className="absolute inset-0 flex items-center justify-center">
                   <div className="relative" style={{ width: '100%', height: '100%' }}>
                     {detectedWords.map((w, i) => (