chore: add Android project files to version control

Adds build config, Gradle wrapper, resources, and gitignore.
Kotlin sources, manifests and app build.gradle were already tracked.
Excludes build output, .gradle cache, .idea, local.properties, and release APK.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

android/gradle.properties

@@ -0,0 +1,4 @@
+org.gradle.jvmargs=-Xmx2048m -Dfile.encoding=UTF-8
+android.useAndroidX=true
+kotlin.code.style=official
+android.nonTransitiveRClass=true