fix(transfer): stop folder downloads looping until they crash

[Orinks]

Summary

• Recursive folder downloads listed the remote tree with no record of directories already seen. A server that names the current folder (or its parent) as a child — FTP cdir/pdir with a real path, a WebDAV self-href — or an SFTP directory symlink pointing back at an ancestor was walked until the process ran out of memory. That is the crash on downloading a folder, especially a large one: listing never finishes, progress stays at 0%, then the app dies.
• The walker now treats the canonical path as directory identity, skips a child that is the folder already being listed, and SFTP realpath collapses symlink cycles. FTP MLSD drops cdir/pdir regardless of how they are named.
• Tests that would have caught it: a self-child listing, a parent-pointer listing, a symlink-to-ancestor, a 400-deep tree, and an 8,000-file folder, each with a listing budget so an unbounded walk fails the test instead of hanging.

Test plan

• cargo test -p portkeydrop-core --lib (528 passed)
• cargo clippy -p portkeydrop-core --all-targets -- -D warnings
• Download a normal nested folder over SFTP and confirm files land and empty-cycle folders do not hang
• If Joshua can say SFTP vs WebDAV vs FTP and roughly how large the folder was, we can confirm this matches the report; no log is required for the unbounded-listing crash itself