SwiftUI Convenience Layer (Host + UI)

This release adds a SwiftUI “convenience layer” on top of the core StoreKit 2 backend.

What’s new
• SwiftUI bootstrap
• PurchaseKitHost to own a single PurchaseKitManager (@StateObject) and inject it via EnvironmentObject
• View.installPurchaseKit(...) helpers (basic, with optional NetworkService, or custom manager factory)
• Package-ready UI components
• PurchaseKitPurchaseButton with sensible defaults and full customization via title/icon/tint providers
• Default localization keys resolved from Bundle.module
• Toolbar menu + legal actions
• PurchaseKitInfoToolbar with optional Terms/Privacy URLs, Info sheet, Restore, and Manage Subscriptions entry
• PurchaseKitSafariView for in-app web presentation
• Documentation
• Updated usage examples and setup guidance

Notes
• Minimum requirements unchanged (iOS 16+, StoreKit 2)
• No breaking API changes intended (compatible with v0.1.0 core)