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/build.gradle.kts

@@ -0,0 +1,5 @@
+plugins {
+    alias(libs.plugins.android.application) apply false
+    alias(libs.plugins.kotlin.android) apply false
+    alias(libs.plugins.kotlin.compose) apply false
+}