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>
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