feat: support PuTTY PPK key files for SFTP authentication

[Orinks]

Feature Request

Support importing PuTTY PPK (PuTTY Private Key) files directly for SFTP site authentication, similar to WinSCP.

Motivation

Many users have an existing library of PPK files from WinSCP or PuTTY. Requiring them to convert every key to OpenSSH format before using PortkeyDrop is a significant migration barrier. Native PPK support would make switching from WinSCP seamless.

Expected Behavior

• User can select a .ppk file when configuring an SFTP site (same as selecting an OpenSSH key today)
• PortkeyDrop reads and uses the PPK key directly without requiring manual conversion
• Supports both PPK v2 and v3 formats
• Passphrase-protected PPK files prompt for passphrase on connect

Implementation Notes

• paramiko does not natively support PPK files, but ppk2openssh or the puttygen conversion approach (via the cryptography library) can convert PPK → OpenSSH in-memory at connect time
• Alternatively, consider bundling a lightweight PPK parser

[Orinks]

Resolved by the asyncssh migration (PR #44, merged). asyncssh natively supports PPK v2 and v3 — users can select any .ppk file in the key file picker and it works without conversion. No manual OpenSSH conversion needed.