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/Features/Paywall/Views/ProBadge.swift

@@ -0,0 +1,46 @@
+//
+//  ProBadge.swift
+//  SportsTime
+//
+//  Small "PRO" badge indicator for locked features.
+//
+
+import SwiftUI
+
+struct ProBadge: View {
+    var body: some View {
+        Text("PRO")
+            .font(.caption2.bold())
+            .foregroundStyle(.white)
+            .padding(.horizontal, 6)
+            .padding(.vertical, 2)
+            .background(Theme.warmOrange, in: Capsule())
+    }
+}
+
+// MARK: - View Modifier
+
+extension View {
+    /// Adds a small PRO badge overlay to indicate locked feature
+    func proBadge(alignment: Alignment = .topTrailing) -> some View {
+        overlay(alignment: alignment) {
+            if !StoreManager.shared.isPro {
+                ProBadge()
+                    .padding(4)
+            }
+        }
+    }
+}
+
+#Preview {
+    VStack(spacing: 20) {
+        ProBadge()
+
+        // Example usage
+        RoundedRectangle(cornerRadius: 12)
+            .fill(.blue.opacity(0.2))
+            .frame(width: 100, height: 60)
+            .proBadge()
+    }
+    .padding()
+}