fix: standardize trip name display with arrow separators app-wide

- 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>

SportsTime/Features/Polls/Views/PollVotingView.swift

@@ -138,7 +138,7 @@ private struct RankingRow: View {
                 .clipShape(Circle())
 
             VStack(alignment: .leading, spacing: 2) {
-                Text(trip.name)
+                Text(trip.displayName)
                     .font(.headline)
 
                 Text(tripSummary)