feat(store): add In-App Purchase system with Pro subscription

Implement freemium model with StoreKit 2:
- StoreManager singleton for purchase/restore/entitlements
- ProFeature enum defining gated features
- PaywallView and OnboardingPaywallView for upsell UI
- ProGate view modifier and ProBadge component

Feature gating:
- Trip saving: 1 free trip, then requires Pro
- PDF export: Pro only with badge indicator
- Progress tab: Shows ProLockedView for free users
- Settings: Subscription management section

Also fixes pre-existing test issues with StadiumVisit
and ItineraryOption model signature changes.

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

SportsTime/SportsTimeApp.swift

@@ -10,6 +10,14 @@ import SwiftData
 
 @main
 struct SportsTimeApp: App {
+    /// Task that listens for StoreKit transaction updates
+    private var transactionListener: Task<Void, Never>?
+
+    init() {
+        // Start listening for transactions immediately
+        transactionListener = StoreManager.shared.listenForTransactions()
+    }
+
     var sharedModelContainer: ModelContainer = {
         let schema = Schema([
             // User data models
@@ -63,6 +71,11 @@ struct BootstrappedContentView: View {
     @State private var isBootstrapping = true
     @State private var bootstrapError: Error?
     @State private var hasCompletedInitialSync = false
+    @State private var showOnboardingPaywall = false
+
+    private var shouldShowOnboardingPaywall: Bool {
+        !UserDefaults.standard.bool(forKey: "hasSeenOnboardingPaywall") && !StoreManager.shared.isPro
+    }
 
     var body: some View {
         Group {
@@ -76,6 +89,15 @@ struct BootstrappedContentView: View {
                 }
             } else {
                 HomeView()
+                    .sheet(isPresented: $showOnboardingPaywall) {
+                        OnboardingPaywallView(isPresented: $showOnboardingPaywall)
+                            .interactiveDismissDisabled()
+                    }
+                    .onAppear {
+                        if shouldShowOnboardingPaywall {
+                            showOnboardingPaywall = true
+                        }
+                    }
             }
         }
         .task {
@@ -109,10 +131,14 @@ struct BootstrappedContentView: View {
             // 3. Load data from SwiftData into memory
             await AppDataProvider.shared.loadInitialData()
 
-            // 4. App is now usable
+            // 4. Load store products and entitlements
+            await StoreManager.shared.loadProducts()
+            await StoreManager.shared.updateEntitlements()
+
+            // 5. App is now usable
             isBootstrapping = false
 
-            // 5. Background: Try to refresh from CloudKit (non-blocking)
+            // 6. Background: Try to refresh from CloudKit (non-blocking)
             Task.detached(priority: .background) {
                 await self.performBackgroundSync(context: context)
                 await MainActor.run {