chore(example): adopt the UIScene lifecycle in the example app

[luflow]

Flutter 3.44 applies this migration to the example project on every build, so anyone building the example finds their working tree modified afterwards. Committing it stops that drift and moves the app onto the scene-based lifecycle iOS expects.

• Info.plist declares a UIApplicationSceneManifest pointing at FlutterSceneDelegate (plus CADisableMinimumFrameDurationOnPhone, which the same migration adds)
• AppDelegate registers plugins through FlutterImplicitEngineDelegate.didInitializeImplicitFlutterEngine instead of doing it in didFinishLaunchingWithOptions

Testing

Built and run on an iPhone 17 Pro simulator (iOS 26.2) — the app launches and renders normally. A lifecycle migration can compile fine and still come up to a blank window, so I checked the running app rather than just the build.

Worth knowing before merging

• Generated with Flutter 3.44.6 (stable). FlutterSceneDelegate and FlutterImplicitEngineDelegate come from that tooling; on an older Flutter this Info.plist would reference classes the engine does not provide.
• Convenience only — you get the same result by building the example yourself. Close it if you would rather not pin the example to that Flutter version yet.
• Independent of chore(example): move the example app off CocoaPods to Swift Package Manager #150 (CocoaPods → SwiftPM) and of feat: section titles in menus, and menus on app bar actions #149; the three can land in any order.