What changed

• Added a yearly subscription option (with optional free trial) as an alternative to the existing one-time purchase on Google Play
• The upgrade screen now shows both options with pricing so users can choose their preferred billing method
• FOSS version now nudges users to actually visit the sponsor page before unlocking features, with a friendly reminder if they return too quickly
• Added "No ads. No tracking. No Google Play lock-in." subtitle to the FOSS sponsor screen

Technical Context

• Rewrote the SKU type system from a flat data class to an interface hierarchy (Sku.Iap, Sku.Subscription) to cleanly support both billing models with type-safe offer matching
• Subscription offers (base plan, trial) are matched by plan ID against Google Play's subscriptionOfferDetails, following the same pattern used in sdmaid-se/octi
• FOSS sponsor gate uses SystemClock.elapsedRealtime() + SavedStateHandle to survive process death and prevent clock manipulation
• Converted gplay UpgradeViewModel.state to StateFlow via stateIn to avoid re-querying billing on config changes
• Removed Activity references from BillingEvent channel to prevent stale activity leaks across config changes