ci: Add GitHub Actions workflow to build split APKs for x86_64 and armeabi-v7a

[Copilot]

No CI pipeline existed to build the Android app. Adds a workflow that triggers on every push and pull request, producing separate debug APKs per ABI.

Changes

.github/workflows/build.yml

• Triggers on push and pull_request across all branches
• Sets up JDK 17 (Temurin) with Gradle cache
• Runs assembleDebug and uploads two artifacts:
  • app-x86_64-debug
  • app-armeabi-v7a-debug

app/build.gradle.kts

• Adds splits.abi block to generate per-ABI APKs instead of a fat APK:

splits {
    abi {
        isEnable = true
        reset()
        include("x86_64", "armeabi-v7a")
        isUniversalApk = false
    }
}