fix: update notification card widgets

[edgarfgp]

Fixes #292 (comment)

• IsClosed is a completion state that triggers NotificationClosed event; it doesn’t itself hide the card.
• To close visually using the control’s own behaviour, you must call Close() (drive IsClosing).
• If you only need to show/hide in the sample, prefer .isVisible(...) or conditional rendering.

So let's remove IsClosed modifier as it is not meant to be used to close the card. Also change NotificationClosed to be a RoutedEvent.

[Copilot]

Pull Request Overview

This PR fixes the NotificationCard API by removing the confusing IsClosed parameter and properly handling the NotificationClosed event as a routed event.

• Removes the IsClosed parameter from NotificationCard constructors, as it doesn't control visual state
• Converts NotificationClosed from a regular event to a RoutedEvent using defineRoutedEvent
• Updates the sample to use conditional rendering instead of the removed IsClosed parameter

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/Fabulous.Avalonia/Views/Controls/Notifications/NotificationCard.fs	Removed IsClosed attribute and simplified NotificationCard builders to remove the confusing isClosed parameter
src/Fabulous.Avalonia/Views/Controls/Notifications/NotificationCard.Mvu.fs	Changed NotificationClosed from defineEvent to defineRoutedEvent for proper routed event handling
src/Fabulous.Avalonia/Views/Controls/Notifications/NotificationCard.Components.fs	Changed NotificationClosed from defineEvent to defineRoutedEvent for proper routed event handling
samples/Gallery/Pages/NotificationsPage.fs	Updated sample to use conditional rendering instead of IsClosed, removed obsolete TODO comment, and minor cleanup

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