Adds a versionCodeOverride to build.gradle.kts (F-Droid requires distinct
versionCodes per split APK; Google Play's App Bundle is unaffected since
this only touches the application-variant/APK output). Bumps to 0.1.2+4.
Verified locally: flutter build apk --release --split-per-abi produces
app-{armeabi-v7a,arm64-v8a,x86_64}-release.apk with versionCode 31/32/33.
