feat: import connection profiles from other FTP/SFTP clients

[Orinks]

User request

Allow importing saved connection profiles from popular FTP/SFTP clients so users don't have to re-enter all their servers when switching to PortkeyDrop.

Clients to support

FileZilla (highest priority — most common)

• Format: XML
• Windows: %APPDATA%\FileZilla\sitemanager.xml
• Linux/macOS: ~/.config/filezilla/sitemanager.xml
• Contains: host, port, protocol (FTP/SFTP/FTPS), username, password (base64, not encrypted), remote path, notes

WinSCP

• Format: INI file (exportable) or Windows Registry
• Export path: File > Export/Backup Configuration > saves .ini
• Registry: HKCU\Software\Martin Prikryl\WinSCP 2\Sessions\
• Contains: host, port, protocol, username, remote directory, key file path

Cyberduck / Mountain Duck

• Format: Per-bookmark .duck files (XML-based)
• Windows: %APPDATA%\Cyberduck\Bookmarks\
• macOS: ~/Library/Application Support/Cyberduck/Bookmarks/
• Contains: host, port, protocol, username, nickname, path

UI

• New menu item: File > Import Connections...
• Wizard-style dialog:
  1. Choose source client (radio buttons: FileZilla / WinSCP / Cyberduck / From file...)
  2. Auto-detect config file location; allow manual browse if not found
  3. Preview list of found connections (checkboxes to select which to import)
  4. Import selected — add to Site Manager, passwords go through the normal three-tier storage
• Show count of successfully imported connections on completion

Notes

• FileZilla passwords are base64-encoded but not encrypted — import them and immediately migrate to keyring/vault via existing _PasswordBackend
• WinSCP registry reading is Windows-only; INI import works cross-platform
• Skip duplicate detection: if a site with the same host+port+username already exists, warn the user rather than creating a duplicate
• Parsers should live in src/portkeydrop/importers/ (one module per client)
• Add tests for each parser using sample config fixtures

[Orinks]

Closing as resolved by merged importer work: PR #23 (core importer framework), plus follow-up fixes in #77 and #79.