fix(ios): adopt the UIScene life cycle — 1.2.3 (27)

Build 26 was the first one made with the iOS 27 SDK, and UIKit stops apps
without the scene life cycle at launch on iOS 27 (TN3187). The scene
delegate only shows the FlutterEngine that AppDelegate starts with the
process, so a VoIP push in the background still finds the CallKit plugin
and a running Dart isolate when no scene is connected.

app/ios/Runner/AppDelegate.swift

@@ -7,6 +7,12 @@ import flutter_callkit_incoming
 
 @main
 @objc class AppDelegate: FlutterAppDelegate, PKPushRegistryDelegate, CallkitIncomingAppDelegate {
+  // Engine startuje s procesem, ne se scénou. VoIP push umí appku spustit na
+  // pozadí, aniž by iOS připojil scénu (a tím vytvořil FlutterViewController).
+  // CallKit plugin musí být zaregistrovaný dřív, než push dorazí, a Dart musí
+  // běžet, aby vyzvedl nabídku hovoru z relaye. SceneDelegate tenhle engine
+  // jen zobrazí.
+  let flutterEngine = FlutterEngine(name: "sepot")
   // PushKit registry musí žít po celou dobu běhu appky, jinak VoIP pushe nechodí.
   private var voipRegistry: PKPushRegistry?
   private var apnsChannel: FlutterMethodChannel?
@@ -17,31 +23,24 @@ import flutter_callkit_incoming
     _ application: UIApplication,
     didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
   ) -> Bool {
-    // Bez force-castu: kdyby root nebyl FlutterViewController (jiný storyboard,
-    // budoucí scény), appka by spadla hned při startu. Registrar dá messenger
-    // enginu i tak.
-    let messenger =
-      (window?.rootViewController as? FlutterViewController)?.binaryMessenger
-      ?? registrar(forPlugin: "SepotApns")?.messenger()
-    if let messenger {
-      let channel = FlutterMethodChannel(
-        name: "cz.sepot.sepot/apns", binaryMessenger: messenger)
-      channel.setMethodCallHandler { [weak self] call, result in
-        switch call.method {
-        case "register":
-          // Nativní APNs registrace (bez Firebase) — token přijde v callbacku
-          application.registerForRemoteNotifications()
-          result(nil)
-        case "getToken":
-          result(self?.pendingToken)
-        default:
-          result(FlutterMethodNotImplemented)
-        }
-      }
-      apnsChannel = channel
-    }
+    flutterEngine.run()
+    GeneratedPluginRegistrant.register(with: flutterEngine)
 
-    GeneratedPluginRegistrant.register(with: self)
+    let channel = FlutterMethodChannel(
+      name: "cz.sepot.sepot/apns", binaryMessenger: flutterEngine.binaryMessenger)
+    channel.setMethodCallHandler { [weak self] call, result in
+      switch call.method {
+      case "register":
+        // Nativní APNs registrace (bez Firebase) — token přijde v callbacku
+        UIApplication.shared.registerForRemoteNotifications()
+        result(nil)
+      case "getToken":
+        result(self?.pendingToken)
+      default:
+        result(FlutterMethodNotImplemented)
+      }
+    }
+    apnsChannel = channel
 
     // Hovory: VoIP push z relaye → CallKit.
     let registry = PKPushRegistry(queue: DispatchQueue.main)