System notifications (and optional tray icon)

[codymullins]

Problem

Browsers raise OS notifications (downloads, Web Notifications API). Skyline has no notification or tray API.

Current state

• No notification/tray API in AppHost/AppWindow.

What's needed

• AppHost.ShowNotificationAsync(title, body, options); optional system-tray icon support.
• macOS UNUserNotificationCenter; Windows toast; Linux desktop-portal/D-Bus notifications.

Acceptance criteria

• A consumer can raise a native notification; clicking it can activate the app.

Notes

May be out of scope for a window-host library; revisit when the Web Notifications API is implemented.