Enable Firebase by default for Android and iOS builds so normal local, test, and production builds no longer depend on passing -PfirebaseEnabled.

Opt out with -PfirebaseEnabled=false or FIREBASE_ENABLED=false. A missing google-services.json or GoogleService-Info.plist now fails the build instead of silently skipping Firebase.

BUILDING.md

@@ -1474,9 +1474,11 @@ So, to create a signed `aab` release:
 
 ```bash
 # if you use pkgx: pkgx +openjdk.org /bin/zsh
-./gradlew bundleRelease -Dlint.baselines.continue=true -PfirebaseEnabled
+./gradlew bundleRelease -Dlint.baselines.continue=true
 ```
 
+Firebase is on by default (`firebaseEnabled=true` in `android/gradle.properties`). Do not pass `-PfirebaseEnabled` for a normal release. F-Droid and other builds that must omit Firebase use `-PfirebaseEnabled=false`.
+
 ... and find your `aab` file at app/build/outputs/bundle/release
 
 * Gradle packages whatever web bundle was last synced into
@@ -1486,8 +1488,6 @@ So, to create a signed `aab` release:
 * `build-android.sh` runs `./gradlew clean`, which deletes any earlier `aab`, so
   run `bundleRelease` after the build script finishes.
 
-* Note that F-Droid builds should omit `-PfirebaseEnabled`.
-
 At play.google.com/console:
 
 - Go to Production or the Closed Testing and either Create Track or Manage Track.