Calendar export and per-item calendar links

Whole-trip export — click the calendar icon in the trip header to download a .ics file containing all flights, hotels, and POIs. This file can be imported into any calendar app (Google Calendar, Apple Calendar, Outlook).

Per-item links — hover over any flight, hotel check-in, hotel check-out, or POI in the timeline to reveal a Google Calendar icon. Clicking it opens a pre-filled Google Calendar event creation page for that specific item.