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")
-    }
 }

composeApp/src/iosMain/kotlin/ch/parano/myice/auth/IosOAuthClient.kt

@@ -0,0 +1,67 @@
+package ch.parano.myice.auth
+
+import kotlinx.cinterop.ExperimentalForeignApi
+import platform.AuthenticationServices.ASWebAuthenticationPresentationContextProvidingProtocol
+import platform.AuthenticationServices.ASWebAuthenticationSession
+import platform.Foundation.NSError
+import platform.Foundation.NSURL
+import platform.UIKit.UIApplication
+import platform.UIKit.UIWindow
+import platform.UIKit.UIWindowScene
+import platform.darwin.NSObject
+import kotlin.coroutines.resume
+import kotlin.coroutines.suspendCoroutine
+
+@OptIn(ExperimentalForeignApi::class)
+class IosOAuthClient : OAuthClient {
+
+    override fun redirectUri(): String = "myice://callback"
+
+    override suspend fun authenticate(loginUrl: String): String? = suspendCoroutine { continuation ->
+        val nsUrl = NSURL.URLWithString(loginUrl) ?: run {
+            continuation.resume(null)
+            return@suspendCoroutine
+        }
+
+        val session = ASWebAuthenticationSession(
+            nsUrl,
+            "myice",
+            completionHandler = { callbackURL: NSURL?, error: NSError? ->
+                if (error != null || callbackURL == null) {
+                    continuation.resume(null)
+                } else {
+                    val urlStr = callbackURL.absoluteString ?: ""
+                    val fragmentIndex = urlStr.indexOf("#")
+                    if (fragmentIndex < 0) {
+                        continuation.resume(null)
+                    } else {
+                        val fragment = urlStr.substring(fragmentIndex + 1)
+                        val params = fragment.split("&").associate { pair ->
+                            val idx = pair.indexOf("=")
+                            if (idx >= 0) pair.substring(0, idx) to pair.substring(idx + 1)
+                            else pair to ""
+                        }
+                        continuation.resume(params["access_token"])
+                    }
+                }
+            }
+        )
+
+        session.presentationContextProvider = object : NSObject(),
+            ASWebAuthenticationPresentationContextProvidingProtocol {
+            override fun presentationAnchorForWebAuthenticationSession(
+                session: ASWebAuthenticationSession
+            ): UIWindow {
+                return keyWindow()
+            }
+        }
+
+        session.start()
+    }
+
+    private fun keyWindow(): UIWindow {
+        val scenes = UIApplication.sharedApplication.connectedScenes
+        val windowScene = scenes.firstOrNull { it is UIWindowScene } as? UIWindowScene
+        return (windowScene?.windows?.firstOrNull() as? UIWindow) ?: UIWindow()
+    }
+}