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/androidMain/kotlin/ch/parano/myice/OAuthCallbackActivity.kt

@@ -0,0 +1,17 @@
+package ch.parano.myice
+
+import android.app.Activity
+import android.net.Uri
+import android.os.Bundle
+import ch.parano.myice.auth.OAuthCallbackHolder
+
+class OAuthCallbackActivity : Activity() {
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+        val uri: Uri? = intent?.data
+        if (uri != null) {
+            OAuthCallbackHolder.handleCallback(uri)
+        }
+        finish()
+    }
+}