Close with Delete or backspace keys

### Problem

We can navigate the indicator by keyboard alright, but there's no way to close a notification

### Proposal

Add a close shortcut with Delete or Backspace

### Prior Art (Optional)

_No response_

Start by locating the notification indicator's existing keyboard navigation and the handler for closing a notification. Add Delete and Backspace as close shortcuts, then verify that both keys close the focused notification without breaking current navigation behavior.

Written by the indexing model from the issue text.