refactor: optimize SFTP file transfers using std::io::copy

[Cleboost]

This pull request refactors the SFTP file transfer logic to simplify the code and improve efficiency by using std::io::copy for file transfers instead of manual buffer management. It also updates the synchronous download function to take ownership of its parameters and a Tokio runtime handle, aligning its API with async best practices. The file explorer UI is updated to use the new function signature.

SFTP file transfer improvements:

• Replaced manual read/write buffer loops with std::io::copy in both upload_file, download_file, and download_file_sync for more concise and efficient file transfer code. [1] [2]
• Updated download_file_sync to accept a Tokio runtime handle and take ownership of its parameters, improving its API and thread safety.

UI integration updates:

• Modified the file explorer to pass the current Tokio runtime handle and owned parameters to the new download_file_sync signature, ensuring compatibility with the refactored SFTP logic.