Make stamp preview content private (#5392)

# Description of Changes
Currently when applying stamps, the preview of the PDF is not being
treated as private content. This correctly marks the content as private

frontend/src/core/components/tools/addStamp/StampPreview.tsx

@@ -4,6 +4,7 @@ import { pdfWorkerManager } from '@app/services/pdfWorkerManager';
 import { useThumbnailGeneration } from '@app/hooks/useThumbnailGeneration';
 import { A4_ASPECT_RATIO, getFirstSelectedPage, getFontFamily, computeStampPreviewStyle, getAlphabetPreviewScale } from '@app/components/tools/addStamp/StampPreviewUtils';
 import styles from '@app/components/tools/addStamp/StampPreview.module.css';
+import {PrivateContent} from "@app/components/shared/PrivateContent";
 
 type Props = {
   parameters: AddStampParameters;
@@ -263,12 +264,14 @@ export default function StampPreview({ parameters, onParameterChange, file, show
         onPointerUp={handlePointerUp}
       >
         {pageThumbnail && (
+          <PrivateContent>
             <img
               src={pageThumbnail}
               alt="page preview"
               className={styles.pageThumbnail}
               draggable={false}
             />
+          </PrivateContent>
         )}
         {parameters.stampType === 'text' && (
           <div