feat: UNO Kotlin Multiplatform app (Android, Desktop, Web)

- Full UNO game logic with AI opponents
- Compose Multiplatform UI with animations
- Shared module (commonMain) with game engine
- Desktop app (Compose Desktop)
- Android app (Compose Multiplatform)
- Web app (Kotlin/Wasm)
- Nix shell for reproducible dev environment

shell.nix

@@ -0,0 +1,61 @@
+{
+  pkgs ? import <nixpkgs> { config.allowUnfree = true; },
+}:
+
+let
+  androidSdk = pkgs.androidenv.composeAndroidPackages {
+    buildToolsVersions = [ "35.0.0" ];
+    platformVersions = [ "35" ];
+    abiVersions = [ "x86_64" ];
+    includeEmulator = false;
+    includeSystemImages = false;
+    includeNDK = false;
+  };
+in
+pkgs.mkShell {
+  buildInputs = with pkgs; [
+    jdk21
+    kotlin
+    gradle
+    androidSdk.androidsdk
+    android-tools
+
+    libGL
+    libX11
+    libXcursor
+    libXext
+    libXi
+    libXrender
+    fontconfig
+    libxkbcommon
+  ];
+
+  shellHook = ''
+    ln -sfn ${pkgs.jdk21}/lib/openjdk .jdk
+    export ANDROID_HOME="${androidSdk.androidsdk}/libexec/android-sdk"
+    export ANDROID_SDK_ROOT="${androidSdk.androidsdk}/libexec/android-sdk"
+    export GRADLE_OPTS="-Dorg.gradle.java.installations.paths=${pkgs.jdk21}/lib/openjdk"
+    export PATH="$ANDROID_HOME/platform-tools:$ANDROID_HOME/tools/bin:$JAVA_HOME/bin:$PATH"
+    export LD_LIBRARY_PATH=${
+      pkgs.lib.makeLibraryPath [
+        pkgs.libGL
+        pkgs.libX11
+        pkgs.libXcursor
+        pkgs.libXext
+        pkgs.libXi
+        pkgs.libXrender
+        pkgs.fontconfig
+        pkgs.libxkbcommon
+      ]
+    }
+
+    echo "sdk.dir=$ANDROID_HOME" > local.properties
+
+    echo "=== UNO KMP Dev Shell ==="
+    echo "JDK:       $(java -version 2>&1 | head -1)"
+    echo "Kotlin:    $(kotlin -version 2>&1 | head -1)"
+    echo "Gradle:    $(gradle --version 2>&1 | grep 'Gradle')"
+    echo "Android:   $ANDROID_HOME"
+    echo "========================"
+  '';
+}