Rewrite from Python to Go for single-binary cross-platform builds

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>

.gitignore

@@ -1,8 +1,10 @@
-# Python
-__pycache__/
-*.py[cod]
-.venv/
-venv/
+# Go binaries
+imapdown
+imapdown-*
+*.exe
+go.sum
 
-# Downloads
-download/
+# Test downloads
+*.eml
+*.zip
+.imapdown_state.json