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/build.gradle.kts

@@ -59,6 +59,7 @@ kotlin {
         val desktopMain by getting {
             dependencies {
                 implementation(libs.ktor.client.cio)
+                implementation(libs.kotlinx.coroutines.swing)
                 implementation(compose.desktop.currentOs)
             }
         }