feat(sharing): implement unified sharing system for social media

Replace old ProgressCardGenerator with protocol-based sharing architecture
supporting trips, achievements, and stadium progress. Features 8 color
themes, Instagram Stories optimization (1080x1920), and reusable card
components with map snapshots.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

SportsTime/Features/Progress/ViewModels/ProgressViewModel.swift

@@ -108,6 +108,12 @@ final class ProgressViewModel {
         leagueProgress.stadiumsRemaining
     }
 
+    /// Count of trips for the selected sport (stub - can be enhanced)
+    var tripCount: Int {
+        // TODO: Fetch saved trips count from SwiftData
+        0
+    }
+
     /// Recent visits sorted by date
     var recentVisits: [VisitSummary] {
         visits