Pin minSdk = 24 explicitly on both apps

Both apps were inheriting `flutter.minSdkVersion`, which currently resolves
to 24 (Android 7.0) in this Flutter SDK but could drift if Flutter bumps
its default. Per product requirement we support Android 7+; making the
floor explicit so it doesn't move silently.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

client_app/android/app/build.gradle.kts

@@ -28,7 +28,7 @@ android {
         applicationId = "com.halobestie.client.client_app"
         // You can update the following values to match your application needs.
         // For more information, see: https://flutter.dev/to/review-gradle-config.
-        minSdk = flutter.minSdkVersion
+        minSdk = 24
         targetSdk = flutter.targetSdkVersion
         versionCode = flutter.versionCode
         versionName = flutter.versionName