feat(crashlytics): report release crashes to Firebase Crashlytics

Routes Flutter framework errors (FlutterError.onError) and uncaught
async/platform errors (PlatformDispatcher.onError) to Crashlytics, but
only in release builds — debug keeps its red error screens and console
traces, and collection is disabled via setCrashlyticsCollectionEnabled
(kReleaseMode) so development crashes never reach the dashboard. Adds
the Crashlytics Gradle plugin alongside the existing google-services
FlutterFire config. iOS dSYM upload run script still to be added in
Xcode (symbolication only; crash capture works without it).

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

android/app/build.gradle.kts

@@ -5,6 +5,7 @@ plugins {
     id("com.android.application")
     // START: FlutterFire Configuration
     id("com.google.gms.google-services")
+    id("com.google.firebase.crashlytics")
     // END: FlutterFire Configuration
     id("kotlin-android")
     // The Flutter Gradle Plugin must be applied after the Android and Kotlin Gradle plugins.