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/build.gradle.kts

@@ -76,7 +76,7 @@ kotlin {
 
 android {
     namespace = "ch.parano.myice"
-    compileSdk = 35
+    compileSdk = 36
 
     defaultConfig {
         applicationId = "ch.parano.myice"