Two bugs in the export_ics path:
1. X-WR-CALNAME was written raw: calendar names containing commas,
semicolons or backslashes produced invalid ICS (RFC 5545 §3.3.11
requires those characters to be escaped as \, \; and \\).
Fix: wrap cal.name in the existing _ics_escape() helper, which is
already used for SUMMARY, DESCRIPTION, and LOCATION on the lines
immediately below.
2. DTSTART and DTEND on non-all-day events always emitted the naive
ISO string (e.g. 20260602T100000) regardless of CalendarEvent.is_utc.
Consumers treat a naive datetime as floating/local time, so UTC
events imported into Google Calendar or Apple Calendar shifted by
the user's timezone offset. Fix: append 'Z' when is_utc is True,
matching the pattern already used by the serialise_event() helper
at line 408.
