feat: implement annotation flattening functionality and add tests

src/js/workflow/nodes/flatten-node.ts

@@ -4,6 +4,7 @@ import { pdfSocket } from '../sockets';
 import type { SocketData } from '../types';
 import { requirePdfInput, processBatch } from '../types';
 import { PDFDocument } from 'pdf-lib';
+import { flattenAnnotations } from '../../utils/flatten-annotations.js';
 
 export class FlattenNode extends BaseWorkflowNode {
   readonly category = 'Secure PDF' as const;
@@ -32,6 +33,12 @@ export class FlattenNode extends BaseWorkflowNode {
           console.error('Flatten form error (may have no forms):', err);
         }
 
+        try {
+          flattenAnnotations(pdfDoc);
+        } catch (err) {
+          console.error('Flatten annotations error:', err);
+        }
+
         const pdfBytes = await pdfDoc.save();
         return {
           type: 'pdf',