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/desktopMain/kotlin/ch/parano/myice/Main.kt

@@ -1,11 +1,11 @@
 package ch.parano.myice
 
-import androidx.compose.material3.Text
 import androidx.compose.ui.window.Window
 import androidx.compose.ui.window.application
+import ch.parano.myice.auth.DesktopOAuthClient
 
 fun main() = application {
     Window(onCloseRequest = ::exitApplication, title = "MyIce") {
-        Text("MyIce")
+        App(oauthClient = DesktopOAuthClient())
     }
 }