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

desktopApp/build.gradle.kts

@@ -0,0 +1,24 @@
+plugins {
+    alias(libs.plugins.kotlinMultiplatform)
+    alias(libs.plugins.composeMultiplatform)
+    alias(libs.plugins.composeCompiler)
+}
+
+kotlin {
+    jvm()
+
+    sourceSets {
+        jvmMain.dependencies {
+            implementation(project(":shared"))
+            implementation(compose.desktop.currentOs)
+            implementation(compose.material3)
+            implementation(compose.foundation)
+        }
+    }
+}
+
+compose.desktop {
+    application {
+        mainClass = "com.uno.desktop.MainKt"
+    }
+}