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/StoreErrorTests.swift

@@ -0,0 +1,16 @@
+//
+//  StoreErrorTests.swift
+//  SportsTimeTests
+//
+
+import Testing
+import Foundation
+@testable import SportsTime
+
+struct StoreErrorTests {
+    @Test func errorDescription_returnsUserFriendlyMessage() {
+        #expect(StoreError.productNotFound.localizedDescription.contains("not found"))
+        #expect(StoreError.purchaseFailed.localizedDescription.contains("failed"))
+        #expect(StoreError.verificationFailed.localizedDescription.contains("verify"))
+    }
+}