IAP module refactor, Kotlin 2, Compose (#3)

Refactor billing flow and structure: add initBillingClient helper,
introduce ProductManager cache, harden fetchProducts and requestPurchase
(multi-SKU, empty SKU guard, clearer errors), expose
getAvailableItems(type) and getStorefront, and reorder public/private
methods for readability. Remove legacy ExpoMapMappers in favor of model
toJSON.

Upgrade tooling to Kotlin 2 with Compose plugin (app and library), align
Java 17, and remove deprecated targetSdk from library DSL.

Author: hyochan