F-Droid: ABI-split builds.

Per-architecture APKs (armeabi-v7a, arm64-v8a, x86_64) reduce
per-user download size ~3×. F-Droid distributes the right APK for
your device's CPU automatically. Universal AAB still ships to Play.

Internal restructure for F-Droid maintainer review feedback. No
user-visible feature changes from v0.3.1.
