Linthra 0.1.0-alpha.30 — build & packaging maintenance.

Changed:
- Updated the Android Gradle tooling (Android Gradle Plugin, the Gradle wrapper,
  and Kotlin) so Linthra builds on Java 21 toolchains, including F-Droid's build
  servers. No app features, permissions, or behavior change.

Still an alpha — expect rough edges. Not on F-Droid yet.
