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"
+    }
+}

desktopApp/src/jvmMain/kotlin/com/uno/desktop/Main.kt

@@ -0,0 +1,94 @@
+package com.uno.desktop
+
+import androidx.compose.foundation.layout.*
+import androidx.compose.material3.*
+import androidx.compose.runtime.*
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.DpSize
+import androidx.compose.ui.unit.dp
+import androidx.compose.ui.window.Window
+import androidx.compose.ui.window.WindowState
+import androidx.compose.ui.window.application
+import com.uno.game.GameEngine
+import com.uno.game.model.*
+import com.uno.shared.UnoGameScreen
+
+fun main() = application {
+    Window(
+        onCloseRequest = ::exitApplication,
+        title = "UNO - Kotlin Multiplatform",
+        state = WindowState(size = DpSize(900.dp, 700.dp))
+    ) {
+        UnoDesktopApp()
+    }
+}
+
+@Composable
+fun UnoDesktopApp() {
+    val engine = remember { GameEngine() }
+    var gameState by remember { mutableStateOf(engine.startGame()) }
+    var gameStarted by remember { mutableStateOf(false) }
+
+    if (!gameStarted) {
+        DesktopStartScreen(onStartGame = { playerCount ->
+            gameState = engine.startGame(playerCount)
+            gameStarted = true
+        })
+    } else {
+        UnoGameScreen(
+            state = gameState,
+            onPlayCard = { card, color ->
+                gameState = engine.playCard(gameState, 0, card, color)
+                gameState = runAiTurns(engine, gameState)
+            },
+            onDrawCard = {
+                gameState = engine.drawCard(gameState, 0)
+                gameState = runAiTurns(engine, gameState)
+            },
+            onCallUno = { gameState = engine.callUno(gameState, 0) }
+        )
+
+        if (gameState.isFinished) {
+            DesktopGameOverScreen(
+                winner = gameState.winner,
+                onPlayAgain = { gameState = engine.startGame() }
+            )
+        }
+    }
+}
+
+private fun runAiTurns(engine: GameEngine, state: GameState): GameState {
+    var currentState = state
+    while (currentState.currentPlayerIndex != 0 && !currentState.isFinished) {
+        currentState = engine.aiPlay(currentState, currentState.currentPlayerIndex)
+    }
+    return currentState
+}
+
+@Composable
+fun DesktopStartScreen(onStartGame: (Int) -> Unit) {
+    Column(
+        modifier = Modifier.fillMaxSize(),
+        horizontalAlignment = Alignment.CenterHorizontally,
+        verticalArrangement = Arrangement.Center
+    ) {
+        Text("UNO", style = MaterialTheme.typography.displayLarge, color = MaterialTheme.colorScheme.primary)
+        Spacer(modifier = Modifier.height(8.dp))
+        Text("Desktop Edition", style = MaterialTheme.typography.titleMedium, color = MaterialTheme.colorScheme.onSurfaceVariant)
+        Spacer(modifier = Modifier.height(24.dp))
+        Button(onClick = { onStartGame(4) }) { Text("Spiel starten (4 Spieler)") }
+    }
+}
+
+@Composable
+fun DesktopGameOverScreen(winner: Player?, onPlayAgain: () -> Unit) {
+    AlertDialog(
+        onDismissRequest = onPlayAgain,
+        title = { Text("Spiel vorbei!") },
+        text = { Text(if (winner != null) "${winner.name} hat gewonnen!" else "Unentschieden!") },
+        confirmButton = {
+            Button(onClick = onPlayAgain) { Text("Nochmal spielen") }
+        }
+    )
+}