feat: add batch conversion support with multi-file upload and ZIP download

Major improvements:
- Updated FileUpload component to support multiple file selection
  - Added drag-and-drop for multiple files
  - Individual file removal
  - "Add More Files" button when files are selected
- Modified FileConverter to handle batch conversions
  - Sequential conversion of multiple files
  - Progress tracking for each file individually
  - All files share same output format and settings
- Added batch download functionality
  - Single file: direct download
  - Multiple files: ZIP archive download
  - Uses jszip library for ZIP creation
- Enhanced UI/UX
  - Show count of selected files in convert button
  - Display all conversion jobs with individual previews
  - "Download All" button for completed conversions
  - Conversion status messages show success/failure counts

Dependencies added:
- jszip@3.10.1 for ZIP file creation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

lib/utils/fileUtils.ts

@@ -93,3 +93,22 @@ export function validateFileType(file: File, allowedTypes: string[]): boolean {
     return file.type === type;
   });
 }
+
+/**
+ * Download multiple blobs as a ZIP file
+ */
+export async function downloadBlobsAsZip(files: Array<{ blob: Blob; filename: string }>, zipFilename: string): Promise<void> {
+  const JSZip = (await import('jszip')).default;
+  const zip = new JSZip();
+
+  // Add all files to ZIP
+  files.forEach(({ blob, filename }) => {
+    zip.file(filename, blob);
+  });
+
+  // Generate ZIP blob
+  const zipBlob = await zip.generateAsync({ type: 'blob' });
+
+  // Download ZIP
+  downloadBlob(zipBlob, zipFilename);
+}