- Add `displayName` computed property to Trip model that always
generates city list with " → " separator for consistent display
- Replace all `trip.name` usages with `trip.displayName` in UI files
- Update SuggestedTripsGenerator to use " → " separator
- Update PDFGenerator to use displayName for PDF titles
This ensures all trip names display consistently regardless of when
the trip was created or how the name was originally stored.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>