feat(todo): Snooze on a reminder on iOS, macOS and Windows (ADR 0033, D4)

Apple's systems take actions from categories registered at start: one
for each length a snooze can be, its action in the language the app
starts in, not to the foreground, so that iOS snoozes in the plugin's
isolate with the app closed (AppDelegate registers the plugins for it).
Windows hands back only the button's arguments, which carry the
notification's ID and payload. macOS and Windows launch the app for an
action while it is closed, and the gateway snoozes what launched it as
it starts. Not yet tried on any of the three devices.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/todo/halkyon_todo_flutter/ios/Runner/AppDelegate.swift

@@ -1,6 +1,8 @@
 import Flutter
 import UIKit
 import UserNotifications
+// For FlutterLocalNotificationsPlugin.setPluginRegistrantCallback.
+import flutter_local_notifications
 
 @main
 @objc class AppDelegate: FlutterAppDelegate, FlutterImplicitEngineDelegate {
@@ -14,6 +16,11 @@ import UserNotifications
   }
 
   func didInitializeImplicitFlutterEngine(_ engineBridge: FlutterImplicitEngineBridge) {
+    // A reminder snoozed with the app closed is scheduled again in an
+    // isolate of the plugin's, which needs the plugins too (ADR 0033, D4).
+    FlutterLocalNotificationsPlugin.setPluginRegistrantCallback { (registry) in
+      GeneratedPluginRegistrant.register(with: registry)
+    }
     GeneratedPluginRegistrant.register(with: engineBridge.pluginRegistry)
   }
 }