BUG: Multi-part email composition doesn't handle nested MIME structures correctly

[andrinoff]

Describe the bug

In sender/sender.go, multi-part email composition builds a specific MIME tree structure. Edge cases with nested multipart types may produce invalid emails:

• Reply to multipart/mixed email with inline images and attachments creates deeply nested MIME
• Forward of multipart/alternative with attachment may duplicate content
• PGP-encrypted reply to HTML email has inconsistent part ordering

To reproduce

1. Reply to an email that is multipart/mixed with inline images
2. Add your own attachment
3. Resulting MIME tree may be 4+ levels deep, confusing some mail clients

Expected behavior

Flatten MIME tree where possible. Follow RFC 2046 recommendations for part ordering. Test with common mail clients for compatibility.