Replaces imapdown.py with a multi-file Go implementation using
github.com/emersion/go-imap/v2. All features preserved: SSL/STARTTLS,
incremental UID-based downloads, attachment extraction to zip,
modified UTF-7 folder name decoding, and full-mode safety checks.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
