liveactivity: fix deprecated end(using:dismissalPolicy:) warning (KC-64)

Omitting the content argument on activity.end(dismissalPolicy:)
resolved to the deprecated iOS 16.1 overload. Pass nil explicitly to
bind to end(content:dismissalPolicy:) instead.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

KisaniCalWidgets/StopCountdownIntent.swift

@@ -19,7 +19,7 @@ struct StopCountdownIntent: LiveActivityIntent {
     func perform() async throws -> some IntentResult {
         for activity in Activity<TaskActivityAttributes>.activities
         where activity.attributes.taskID == taskID {
-            await activity.end(dismissalPolicy: .immediate)
+            await activity.end(nil, dismissalPolicy: .immediate)
         }
         return .result()
     }