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

.gitignore

@@ -0,0 +1,26 @@
+# Gradle
+.gradle/
+build/
+local.properties
+*.hprof
+
+# IDE
+.idea/
+*.iml
+*.ipr
+*.iws
+.vscode/
+
+# Nix
+result
+.jdk
+
+# Kotlin
+*.class
+*.jar
+!gradle/wrapper/gradle-wrapper.jar
+
+# OS
+.DS_Store
+Thumbs.db
+.kotlin/