feat(android): split release APK per ABI for F-Droid

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.

CHANGELOG.md

@@ -3,6 +3,13 @@
 All notable changes to Tane are recorded here. Human-readable, newest
 first. Dates are ISO (YYYY-MM-DD).
 
+## [0.1.2] — 2026-07-17
+
+### Changed
+- Release builds now produce one split APK per CPU architecture (smaller
+  downloads) instead of a single universal APK, in addition to the App
+  Bundle Google Play already used. No user-visible change.
+
 ## [0.1.1] — 2026-07-17
 
 ### Changed