I want to attach an iCalendar (.ics) file to the reservation confirmation email so that booked dates can be directly imported into a smartphone calendar.

Requirements:
- Format the outgoing email as a `multipart/mixed` MIME message.
- Dynamically generate an iCalendar (`.ics`) file containing the booking details (date, selected activity/activities, name).
- Set proper MIME headers (`Content-Type: text/calendar`, `Content-Disposition: attachment; filename="date-event.ics"`) so smartphones (iOS / Android) and calendar apps present a direct "Add to Calendar" import prompt.