- BuildCsv per-user CSV with summary section (user, totals, sites, high-privilege, date)
- WriteAsync groups entries by UserLogin, writes one file per user (audit_{email}_{date}.csv)
- WriteSingleFileAsync combines all users in one file for SaveFileDialog export
- RFC 4180 CSV escaping, UTF-8 with BOM for Excel compatibility
- SanitizeFileName strips invalid path chars from email addresses
