Add support for multiple file transfer

## Feature Request

Currently, OpenDrop supports transferring individual files one at a time. This feature request is to add support for transferring multiple files simultaneously.

## Use Case
Users often need to transfer multiple files across devices. The current implementation requires selecting and transferring files one by one, which is time-consuming and inefficient for bulk transfers.

## Proposed Solution
- Allow users to select multiple files at once
- Display a batch transfer interface showing all selected files
- Provide progress tracking for each file in the transfer queue
- Support drag-and-drop for multiple files
- Allow users to manage the transfer queue (add/remove files)

## Expected Behavior
1. User selects multiple files from their file system
2. OpenDrop displays all selected files with their sizes
3. User initiates transfer
4. Progress is shown for each file
5. Transfer completes with summary of all transferred files

## Additional Notes
This would significantly improve the user experience for common use cases like transferring documents, photos, or project files between devices.