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>

package.json

@@ -14,6 +14,7 @@
     "@imagemagick/magick-wasm": "^0.0.30",
     "clsx": "^2.1.1",
     "fuse.js": "^7.1.0",
+    "jszip": "^3.10.1",
     "lucide-react": "^0.553.0",
     "next": "^16.0.0",
     "react": "^19.0.0",