bug(sftp): download stuck at 0% after asyncssh migration

[Orinks]

Description

After migrating from paramiko to asyncssh (#44), file downloads get stuck at 0% and never progress.

Expected Behavior

Download progress should update correctly and complete successfully.

Likely Cause

The asyncssh get() / sftp.getfo() API uses a different progress callback signature than the paramiko approach. The progress reporting likely needs to be rewired to use asyncssh's native progress handler.

Steps to Reproduce

1. Connect to an SFTP site
2. Select a file and initiate a download
3. Progress stays at 0% indefinitely

Notes

• Uploads may be affected too — worth checking
• asyncssh uses progress_handler callback on get()/put() methods

[Orinks]

Fixed by #48 — switched SFTPClient.download() and upload() to use asyncssh's native sftp.get()/sftp.put() with progress_handler callback. Progress now flows correctly through the existing wx.PostEvent thread-safe path. All 311 tests green, ruff clean.