What changed

Fixed a crash that could happen when querying in-app purchases or subscriptions from Google Play.

Technical Context

• Root cause: BillingClientConnection used manual suspendCoroutine callback bridging for billing API calls. Google's Billing Library can occasionally invoke async callbacks more than once (race in BillingClientImpl), and SafeContinuation throws IllegalStateException: Already resumed on the second invocation
• Fix: replaced all three manual callback bridges (queryPurchasesAsync, acknowledgePurchase, queryProductDetailsAsync) with billing-ktx's built-in suspend extension functions, which handle callback bridging internally
• billing-ktx:8.0.0 was already a dependency but wasn't being used for these calls