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/AndroidManifest.xml

@@ -15,6 +15,17 @@
                 <category android:name="android.intent.category.LAUNCHER" />
             </intent-filter>
         </activity>
+        <activity
+            android:name=".OAuthCallbackActivity"
+            android:exported="true"
+            android:launchMode="singleTask">
+            <intent-filter>
+                <action android:name="android.intent.action.VIEW" />
+                <category android:name="android.intent.category.DEFAULT" />
+                <category android:name="android.intent.category.BROWSABLE" />
+                <data android:scheme="myice" android:host="callback" />
+            </intent-filter>
+        </activity>
     </application>
 
 </manifest>

composeApp/src/androidMain/kotlin/ch/parano/myice/MainActivity.kt

@@ -4,6 +4,7 @@ import android.os.Bundle
 import androidx.activity.ComponentActivity
 import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
+import androidx.compose.material3.Text
 
 class MainActivity : ComponentActivity() {
     override fun onCreate(savedInstanceState: Bundle?) {

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()
+    }
+}

composeApp/src/androidMain/kotlin/ch/parano/myice/auth/AndroidOAuthClient.kt

@@ -0,0 +1,39 @@
+package ch.parano.myice.auth
+
+import android.content.Context
+import android.content.Intent
+import android.net.Uri
+import kotlinx.coroutines.CompletableDeferred
+
+class AndroidOAuthClient(private val context: Context) : OAuthClient {
+
+    override fun redirectUri(): String = "myice://callback"
+
+    override suspend fun authenticate(loginUrl: String): String? {
+        OAuthCallbackHolder.deferred = CompletableDeferred()
+
+        val intent = Intent(Intent.ACTION_VIEW, Uri.parse(loginUrl)).apply {
+            addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
+        }
+        context.startActivity(intent)
+
+        val callbackUri = OAuthCallbackHolder.deferred?.await() ?: return null
+
+        val fragment = callbackUri.fragment ?: return null
+        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 ""
+        }
+        return params["access_token"]
+    }
+}
+
+object OAuthCallbackHolder {
+    var deferred: CompletableDeferred<Uri>? = null
+
+    fun handleCallback(uri: Uri) {
+        deferred?.complete(uri)
+        deferred = null
+    }
+}