Replace Slint UI with UIKit and add milestones

Fixes #2.\nFixes #9.

ios/Sources/AppDelegate.swift

@@ -0,0 +1,19 @@
+import UIKit
+
+@main
+final class AppDelegate: UIResponder, UIApplicationDelegate {
+    var window: UIWindow?
+
+    func application(
+        _ application: UIApplication,
+        didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]? = nil
+    ) -> Bool {
+        let window = UIWindow(frame: UIScreen.main.bounds)
+        let context = AppContext(window: window)
+        window.rootViewController = context.makeRootController()
+        window.makeKeyAndVisible()
+        self.window = window
+        context.showStartupErrorIfNeeded()
+        return true
+    }
+}