start kotlin/jetpack compose rewrite

replace the flutter project with a gradle/compose android app:
- nix flake dev shell (jdk21, android sdk 35, build-tools 37, gradle)
- material3 app with a placeholder screen
- roborazzi/robolectric screenshot tests with one golden
- carry over the existing launcher/notification assets

gradle.properties

@@ -0,0 +1,9 @@
+org.gradle.jvmargs=-Xmx4g -Dfile.encoding=UTF-8
+org.gradle.parallel=true
+org.gradle.caching=true
+
+android.useAndroidX=true
+android.nonTransitiveRClass=true
+android.builder.sdkDownload=false
+
+kotlin.code.style=official