Add in-app rating prompt (iOS + Android)

Request an App Store / Play Store review after a successful export
(Overview PDF, BOM PDF, or wiring diagram). A shared gate keeps prompts
rare: >=2 successful exports, >=3 days since install, >=120 days since the
last prompt, and at most once per app version. A one-time migration
backdates existing users so the prompt can fire on their first export
after updating. Logs a "Review Prompt Requested" analytics event.

iOS uses StoreKit's AppStore.requestReview(in:) with UserDefaults state;
Android uses the Play In-App Review API with DataStore state.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

android/gradle/libs.versions.toml

@@ -14,6 +14,7 @@ okhttp = "4.12.0"
 serialization = "1.7.3"
 retrofitSerialization = "1.0.0"
 coil = "2.7.0"
+playReview = "2.0.2"
 
 [libraries]
 androidx-core-ktx = { group = "androidx.core", name = "core-ktx", version.ref = "coreKtx" }
@@ -39,6 +40,7 @@ okhttp = { group = "com.squareup.okhttp3", name = "okhttp", version.ref = "okhtt
 okhttp-logging = { group = "com.squareup.okhttp3", name = "logging-interceptor", version.ref = "okhttp" }
 kotlinx-serialization-json = { group = "org.jetbrains.kotlinx", name = "kotlinx-serialization-json", version.ref = "serialization" }
 coil-compose = { group = "io.coil-kt", name = "coil-compose", version.ref = "coil" }
+play-review-ktx = { group = "com.google.android.play", name = "review-ktx", version.ref = "playReview" }
 
 [plugins]
 android-application = { id = "com.android.application", version.ref = "agp" }