fix(sftp): fix UI freeze and READDIR hang on directories with special files

[Orinks]

Problem

Two related issues caused the app to freeze or hang when navigating remote directories:

1. UI freeze: Directory listing and chdir were running on the main thread, blocking the entire UI
2. Infinite READDIR loop: Some SFTP servers (e.g. Synology NAS) respond to SSH_FXP_READDIR with count=0 instead of SSH_FX_EOF for empty directories. Paramiko loops forever in this case — the app hangs indefinitely

Root Cause

Paramiko's listdir_attr does not handle count=0 as end-of-directory. WinSCP and other clients treat count=0 as EOF; this PR replicates that behaviour via a custom _listdir_attr_safe implementation.

Changes

• Move chdir and listdir_attr onto daemon threads so the UI remains responsive
• Add _listdir_attr_safe: same as sftp.listdir_attr but breaks on count=0
• Add _sftp_lock to serialise SFTP calls (paramiko sessions are not thread-safe)
• Skip special files (sockets, FIFOs, block/char devices) that can hang on stat()
• Add --debug and --log=<file> CLI flags for diagnostic output
• Add tests for threaded refresh, special file skipping, and chdir error handling

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