revenuecat-paywall: display a RevenueCat paywall

Use this skill when the user wants to show a paywall that is built and configured in the RevenueCat dashboard, using the native RevenueCatUI components. This skill does not cover building a custom paywall from scratch. For that, use revenuecat-purchase-flow (when available) and Purchases.getOfferings(…) directly.

Prerequisite: integrate-revenuecat has already run. Purchases.configure(…) must succeed before a paywall can load.

1. Detect the platform

Inspect the working directory and pick the first match, from top to bottom:

1. React Native: package.json has a react-native-purchases entry, or react-native as a dependency. react-native-purchases-ui is the paywall package. Read platforms/react-native.md. If expo is also a dependency, note it as an Expo project.
2. Flutter: pubspec.yaml exists at the project root. The paywall package is purchases_ui_flutter. Read platforms/flutter.md.
3. Kotlin Multiplatform: build.gradle.kts contains a kotlin { … } multiplatform source sets block, or depends on com.revenuecat.purchases:purchases-kmp*. The paywall module is purchases-kmp-ui. Read platforms/kmp.md.
4. Android (native): build.gradle(.kts) applies com.android.application (and is not KMP). The paywall dependency is com.revenuecat.purchases:purchases-ui. Read platforms/android.md.
5. iOS (native): Package.swift, *.xcodeproj, *.xcworkspace, or Podfile at the project root. The paywall product is RevenueCatUI. Read platforms/ios.md.

If several match (e.g. an ios/ folder inside a Flutter project), pick the outermost project, the one that owns the build. If still ambiguous, ask the user which platform they want to configure.

2. Shared concepts (all platforms)