Sync folder with another lab

1. Destination folder path construction — The original code used += instead of = when prepending /, causing the folder name to double (e.g. "myfolder" became "myfolder/myfolder"). Fixed to destination_folder = "/" + destination_folder.

2. Trailing slash on S3 destination — Without a trailing slash, rclone interprets :s3:bucket/path as a file, not a directory prefix. This caused the "is a file not a directory" critical error. Added automatic trailing slash to the destination folder.

3. Critical log level not captured — The JSON log parser only matched "error" level messages, but rclone sends fatal failures as "critical". This meant the real error was silently ignored (logged as "JSON log ignored by parser"). Fixed to match both "error" and "critical" levels.

4. Separated stdout and stderr streams — The original code merged stderr into stdout with stderr=subprocess.STDOUT. Since rclone sends JSON logs to stderr and progress to stdout, this could cause parsing issues. Each stream now has its own reader thread.