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/settings.gradle.kts

@@ -0,0 +1,24 @@
+pluginManagement {
+    repositories {
+        google {
+            content {
+                includeGroupByRegex("com\\.android.*")
+                includeGroupByRegex("com\\.google.*")
+                includeGroupByRegex("androidx.*")
+            }
+        }
+        mavenCentral()
+        gradlePluginPortal()
+    }
+}
+
+dependencyResolutionManagement {
+    repositoriesMode.set(RepositoriesMode.FAIL_ON_PROJECT_REPOS)
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+
+rootProject.name = "Haushalt"
+include(":app")