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

flake.nix

@@ -0,0 +1,58 @@
+{
+  description = "Subtracks - Kotlin/Jetpack Compose Android development shell";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/6b316287bae2ee04c9b93c8c858d930fd07d7338";
+  };
+
+  outputs =
+    { nixpkgs, ... }:
+    let
+      systems = [ "x86_64-linux" ];
+      forEachSystem = nixpkgs.lib.genAttrs systems;
+    in
+    {
+      devShells = forEachSystem (
+        system:
+        let
+          pkgs = import nixpkgs {
+            inherit system;
+            config = {
+              android_sdk.accept_license = true;
+              allowUnfree = true;
+            };
+          };
+          android = pkgs.androidenv.composeAndroidPackages {
+            platformVersions = [ "35" ];
+            buildToolsVersions = [ "latest" ];
+            includeEmulator = false;
+            includeSystemImages = false;
+            includeNDK = false;
+            includeSources = false;
+            includeCmake = false;
+          };
+          androidSdk = android.androidsdk;
+        in
+        {
+          default = pkgs.mkShell {
+            packages = [
+              pkgs.jdk21
+              pkgs.gradle
+              pkgs.git
+              androidSdk
+              android.platform-tools
+            ];
+
+            ANDROID_HOME = "${androidSdk}/libexec/android-sdk";
+            ANDROID_SDK_ROOT = "${androidSdk}/libexec/android-sdk";
+
+            shellHook = ''
+              echo "subtracks dev shell"
+              echo "  java:        $(java -version 2>&1 | head -n1)"
+              echo "  ANDROID_HOME: $ANDROID_HOME"
+            '';
+          };
+        }
+      );
+    };
+}