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

build.gradle.kts

@@ -0,0 +1,8 @@
+plugins {
+    alias(libs.plugins.kotlinMultiplatform) apply false
+    alias(libs.plugins.composeMultiplatform) apply false
+    alias(libs.plugins.composeCompiler) apply false
+    alias(libs.plugins.androidApplication) apply false
+    alias(libs.plugins.androidLibrary) apply false
+    alias(libs.plugins.kotlinSerialization) apply false
+}