feat: wire up app root, navigation, and platform entry points

- Add App.kt root composable with state-based routing
  (Login -> Schedule -> EventDetail) driven by AuthViewModel.state
- Update MainActivity (Android), MainViewController (iOS), and
  Main.kt (Desktop) to host App() with their platform OAuthClient
- Add iOS Xcode source files (MyIceApp.swift, ContentView.swift)
  and Info.plist with myice:// URL scheme
- Delete Placeholder.kt
- Add kotlinx-coroutines-swing to desktopMain to provide
  Dispatchers.Main for viewModelScope (app crashed on desktop
  without it)

composeApp/src/androidMain/kotlin/ch/parano/myice/MainActivity.kt

@@ -4,14 +4,14 @@ import android.os.Bundle
 import androidx.activity.ComponentActivity
 import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
-import androidx.compose.material3.Text
+import ch.parano.myice.auth.AndroidOAuthClient
 
 class MainActivity : ComponentActivity() {
     override fun onCreate(savedInstanceState: Bundle?) {
         enableEdgeToEdge()
         super.onCreate(savedInstanceState)
         setContent {
-            Text("MyIce")
+            App(oauthClient = AndroidOAuthClient(applicationContext))
         }
     }
 }