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/ui.ts

@@ -814,7 +814,7 @@ export const toolTemplates = {
     flatten: () => `
         <h2 class="text-2xl font-bold text-white mb-4">Flatten PDF</h2>
         <p class="mb-6 text-gray-400">Make PDF forms and annotations non-editable by flattening them.</p>
-        ${createFileInputHTML()}
+        ${createFileInputHTML({ multiple: true, showControls: true })}
         <div id="file-display-area" class="mt-4 space-y-2"></div>
         <button id="process-btn" class="hidden mt-6 btn-gradient w-full">Flatten PDF</button>
     `,