add kotlin app setup

Minimal Android app in ./app/ matching `# Setup module`:
- Root project (settings.gradle.kts, build.gradle.kts, libs.versions.toml)
  with AGP 8.7.3, Kotlin 2.0.21, Compose BOM 2024.12.01
- App module :app with Jetpack Compose + Navigation Compose dependency
- MainActivity launches MainScreen (empty Composable, no content)
- Default Material 3 theme scaffolding
- System launcher icon, no custom design yet

Verified via ./gradlew assembleDebug (JDK 17, SDK 35).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/.gitignore

@@ -0,0 +1,12 @@
+*.iml
+.gradle
+.kotlin
+/local.properties
+/.idea
+.DS_Store
+/build
+/captures
+.externalNativeBuild
+.cxx
+local.properties
+app/build