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>

SportsTimeTests/Store/ProFeatureTests.swift

@@ -0,0 +1,35 @@
+//
+//  ProFeatureTests.swift
+//  SportsTimeTests
+//
+
+import Testing
+@testable import SportsTime
+
+struct ProFeatureTests {
+    @Test func allCases_containsExpectedFeatures() {
+        let features = ProFeature.allCases
+        #expect(features.contains(.unlimitedTrips))
+        #expect(features.contains(.pdfExport))
+        #expect(features.contains(.progressTracking))
+        #expect(features.count == 3)
+    }
+
+    @Test func displayName_returnsHumanReadableString() {
+        #expect(ProFeature.unlimitedTrips.displayName == "Unlimited Trips")
+        #expect(ProFeature.pdfExport.displayName == "PDF Export")
+        #expect(ProFeature.progressTracking.displayName == "Progress Tracking")
+    }
+
+    @Test func description_returnsMarketingCopy() {
+        #expect(ProFeature.unlimitedTrips.description.contains("trips"))
+        #expect(ProFeature.pdfExport.description.contains("PDF"))
+        #expect(ProFeature.progressTracking.description.contains("stadium"))
+    }
+
+    @Test func icon_returnsValidSFSymbol() {
+        #expect(!ProFeature.unlimitedTrips.icon.isEmpty)
+        #expect(!ProFeature.pdfExport.icon.isEmpty)
+        #expect(!ProFeature.progressTracking.icon.isEmpty)
+    }
+}