WebDAV already sanitizes server-supplied names, but SFTP passed entry.name
through unfiltered, and the engine had no central guard — a malicious or
compromised remote could return '../../x' and (on the JavaFile backend) write
outside the sync root.
- SyncEngine: isUnsafeSyncPath() rejects empty, absolute, and any '..'-segment
path; every file is checked before any read/write/delete (covers all providers).
- SftpProvider.listFiles: drop '.'/'..' and names containing path separators.
- PathSafetyTest covers traversal, backslash, absolute, and empty cases.
