feat(merge): Migrate merge operations to Web Workers and reorganize tool categories

- Create merge.worker.js and alternate-merge.worker.js for offloading PDF merge operations to background threads
- Migrate merge and alternate-merge logic to use Web Worker architecture for improved performance
- Move flatten tool from singlePdfLoadTools to multiFileTools configuration
- Update merge and alternate-merge tool subtitles to indicate bookmark preservation
- Refactor alternate-merge.ts to use Web Worker instead of pdf-lib for interleaving operations
- Refactor merge.ts to use Web Worker for standard merge operations
- Update fileHandler.ts to recognize flatten as a multi-file tool requiring process button handling
- Simplify form-creator.ts and flatten.ts implementations with Web Worker integration
- Update UI state management to support Web Worker-based PDF processing
- Improve performance by moving computationally intensive PDF operations off the main thread

src/js/logic/flatten.ts

@@ -1,6 +1,7 @@
 import { showLoader, hideLoader, showAlert } from '../ui.js';
-import { downloadFile } from '../utils/helpers.js';
+import { downloadFile, readFileAsArrayBuffer } from '../utils/helpers.js';
 import { state } from '../state.js';
+import { PDFDocument } from 'pdf-lib';
 
 export function flattenFormsInDoc(pdfDoc) {
   const form = pdfDoc.getForm();
@@ -8,30 +9,78 @@ export function flattenFormsInDoc(pdfDoc) {
 }
 
 export async function flatten() {
-  if (!state.pdfDoc) {
-    showAlert('Error', 'PDF not loaded.');
+  if (state.files.length === 0) {
+    showAlert('No Files', 'Please select at least one PDF file.');
     return;
   }
-  showLoader('Flattening PDF...');
-  try {
-    flattenFormsInDoc(state.pdfDoc);
 
-    const flattenedBytes = await state.pdfDoc.save();
-    downloadFile(
-      new Blob([flattenedBytes], { type: 'application/pdf' }),
-      'flattened.pdf'
-    );
+  try {
+    if (state.files.length === 1) {
+      showLoader('Flattening PDF...');
+      const file = state.files[0];
+      const arrayBuffer = await readFileAsArrayBuffer(file);
+      const pdfDoc = await PDFDocument.load(arrayBuffer as ArrayBuffer, { ignoreEncryption: true });
+
+      try {
+        flattenFormsInDoc(pdfDoc);
+      } catch (e) {
+        if (e.message.includes('getForm')) {
+          // Ignore if no form found
+        } else {
+          throw e;
+        }
+      }
+
+      const flattenedBytes = await pdfDoc.save();
+      downloadFile(
+        new Blob([flattenedBytes as any], { type: 'application/pdf' }),
+        `flattened_${file.name}`
+      );
+      hideLoader();
+    } else {
+      showLoader('Flattening multiple PDFs...');
+      const JSZip = (await import('jszip')).default;
+      const zip = new JSZip();
+      let processedCount = 0;
+
+      for (let i = 0; i < state.files.length; i++) {
+        const file = state.files[i];
+        showLoader(`Flattening ${i + 1}/${state.files.length}: ${file.name}...`);
+
+        try {
+          const arrayBuffer = await readFileAsArrayBuffer(file);
+          const pdfDoc = await PDFDocument.load(arrayBuffer as ArrayBuffer, { ignoreEncryption: true });
+
+          try {
+            flattenFormsInDoc(pdfDoc);
+          } catch (e) {
+            if (e.message.includes('getForm')) {
+              // Ignore if no form found
+            } else {
+              throw e;
+            }
+          }
+
+          const flattenedBytes = await pdfDoc.save();
+          zip.file(`flattened_${file.name}`, flattenedBytes);
+          processedCount++;
+        } catch (e) {
+          console.error(`Error processing ${file.name}:`, e);
+        }
+      }
+
+      if (processedCount > 0) {
+        const zipBlob = await zip.generateAsync({ type: 'blob' });
+        downloadFile(zipBlob, 'flattened_pdfs.zip');
+        showAlert('Success', `Processed ${processedCount} PDFs.`);
+      } else {
+        showAlert('Error', 'No PDFs could be processed.');
+      }
+      hideLoader();
+    }
   } catch (e) {
     console.error(e);
-    if (e.message.includes('getForm')) {
-      showAlert(
-        'No Form Found',
-        'This PDF does not contain any form fields to flatten.'
-      );
-    } else {
-      showAlert('Error', 'Could not flatten the PDF.');
-    }
-  } finally {
     hideLoader();
+    showAlert('Error', e.message || 'An unexpected error occurred.');
   }
 }