feat: add platform OAuth implementations (Android, iOS, Desktop)

- AndroidOAuthClient: ACTION_VIEW + OAuthCallbackActivity (myice://callback)
- IosOAuthClient: ASWebAuthenticationSession with presentation context provider
- DesktopOAuthClient: local HTTP server + system browser
- AndroidManifest: register OAuthCallbackActivity intent filter
- Align Kotlin to 2.3.20 (required by Compose MP 1.11.1 klibs) and
  compileSdk to 36 (required by activity-compose 1.13.0); fix missing
  Text import and stray brace in placeholder iOS/Android entry points

composeApp/src/iosMain/kotlin/ch/parano/myice/MainViewController.kt

@@ -1,8 +1,8 @@
 package ch.parano.myice
 
+import androidx.compose.material3.Text
 import androidx.compose.ui.window.ComposeUIViewController
 
 fun MainViewController() = ComposeUIViewController {
     Text("MyIce")
-    }
 }