Add UIScene lifecycle support (#6, #74)

[segidev]

Apple is requiring iOS apps to adopt the UISceneDelegate lifecycle, and Flutter 3.38+ makes it the default for new and migrated apps (see docs.flutter.dev/to/uiscene-migration). Once a host app turns on UIApplicationSceneManifest, the existing application(_:open:) / application(_:didFinishLaunchingWithOptions:) hooks in this plugin are no-ops — share URLs never reach the Dart side.

This PR teaches the plugin to participate in the new scene lifecycle.

What changed

• SwiftFlutterSharingIntentPlugin now conforms to FlutterSceneLifeCycleDelegate and is registered via registrar.addSceneDelegate(instance) alongside the existing addApplicationDelegate(instance) call, so the plugin keeps working under both lifecycles.
• New scene methods mirror the existing AppDelegate hooks:
  • scene(_:willConnectTo:options:) — captures the cold-launch share URL from connectionOptions.urlContexts / userActivities and primes initialSharing.
  • scene(_:openURLContexts:) — handles warm shares.
  • scene(_:continue:) — handles NSUserActivity continuations.
• All three return Bool (true when the plugin consumed the URL) so FlutterSceneDelegate knows it doesn't need to forward the URL to the engine's deep-link channel.
• pubspec.yaml: bumped minimum Flutter to >=3.38.0 and Dart SDK to >=3.10.0 (required for the FlutterSceneLifeCycleDelegate protocol).
• CHANGELOG.md: new 2.1.0 entry.
• README.md: short "UIScene lifecycle (Flutter 3.38+)" section under the iOS setup steps.

Closes

• Scene Delegate #6 Scene Delegate
• Add UIScene life cycle support #74 Add UIScene life cycle support

Migration notes for host apps

The plugin handles share URLs natively under UIScene — no extra wiring needed for that. However, when an app adopts UIScene:

1. Plugin registration must move from application(_:didFinishLaunchingWithOptions:) into didInitializeImplicitFlutterEngine(_:) on a FlutterImplicitEngineDelegate-conforming AppDelegate. window.rootViewController is not available at didFinishLaunching time anymore.
2. If you previously had a custom application(_:open:) override that called SwiftFlutterSharingIntentPlugin.instance.application(...), you can remove it — the new scene(_:openURLContexts:) does the equivalent.
3. Apps with a custom Flutter router (GoRouter, Navigator 2.0, etc.) may still see share-scheme URLs reach the router on some engine versions; the README shows the one-line redirect filter that solves it.

Testing

• Existing Dart tests still pass (flutter test — 4/4 green). No Dart API surface changed.
• Manually verified on iPhone 17 (iOS 26.4.2) + Flutter 3.41.9:
  • Cold launch (app killed) — share PDF from Gmail / Files → app opens, getInitialSharing returns the file, share handler fires.
  • Warm share (app in background) — same flow via getMediaStream.
  • No regression on the legacy UIApplicationDelegate path (verified by leaving addApplicationDelegate in place).

No new native test target was added — UIScene, UISceneSession, and UIOpenURLContext cannot be constructed in XCTest, so the new scene callbacks are only manually verifiable.