Three issues addressed in sendImipReply, sendImipInvitation and
sendImipCancellation:
1. Line folding (RFC 5545 §3.1)
Add foldIcsLine() helper that wraps iCalendar content lines at
74 characters, inserting CRLF + SPACE as required by the spec.
Previously, long lines (e.g. ATTENDEE with a full CN and mailto
URI) could exceed the 75-octet limit and cause strict parsers to
silently reject the ICS.
2. MIME wrapper type (RFC 6047 §3 + CalConnect iMIP Best Practices)
Change bodyStructure from multipart/alternative to multipart/mixed.
The CalConnect interoperability guide recommends multipart/mixed as
the outer wrapper for messages carrying a text/calendar part; many
clients skip iTIP processing when they see multipart/alternative.
3. Calendar part metadata
Add charset=UTF-8 to the text/calendar Content-Type, disposition
inline, and a descriptive filename (reply.ics / invite.ics /
cancel.ics) to each outgoing calendar MIME part.
Note: Gmail-to-Gmail events are handled by Google's internal scheduling
API and cannot be updated via iMIP regardless of MIME structure. This
fix improves interoperability with Outlook, Thunderbird, Fastmail and
standard CalDAV servers.
