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/app/build.gradle.kts

@@ -115,4 +115,6 @@ dependencies {
     implementation(libs.kotlinx.serialization.json)
 
     implementation(libs.coil.compose)
+
+    implementation(libs.play.review.ktx)
 }