revenuecat-customer-center: add the RevenueCat Customer Center

Use this skill when the user wants an out of the box UI that lets their customers manage active subscriptions, request refunds, cancel, restore, or contact support, without shipping custom UI. The UI is configured in the RevenueCat dashboard and rendered by the RevenueCatUI SDKs.

Prerequisite: integrate-revenuecat has already run. Purchases.configure(…) must succeed before the Customer Center can load customer data.

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. The Customer Center ships in react-native-purchases-ui. 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 Customer Center ships in purchases_ui_flutter. Read platforms/flutter.md.
3. Kotlin Multiplatform: build.gradle.kts has a kotlin { … } multiplatform block, or depends on com.revenuecat.purchases:purchases-kmp*. The Customer Center composable is in purchases-kmp-ui. Read platforms/kmp.md.
4. Android (native): build.gradle(.kts) applies com.android.application (and is not KMP). The Customer Center composable is in com.revenuecat.purchases:purchases-ui. Read platforms/android.md.
5. iOS (native): Package.swift, *.xcodeproj, *.xcworkspace, or Podfile at the project root. CustomerCenterView is in 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)