The notebook editor has actions for adding and moving cells but no way
to delete one. This adds `notebook::DeleteCell`:

- Trash icon button in the notebook toolbar (own group below
add-markdown/add-code, disabled when the notebook has no cells)
- Jupyter-style `d d` binding plus `backspace` in cell command mode
(macOS and Linux keymaps)
- Deletes the selected cell; selection moves to the neighboring cell,
focus returns to command mode, and the item is marked dirty via the
existing structural-change tracking so saving persists the deletion

Tested locally on macOS with the notebook feature enabled: button click
and bindings delete the selected cell and saving writes the result.
(Screenshots in comments.)

Release Notes:

- Added a delete-cell action (`d d` in command mode and a toolbar
button) to the notebook editor (preview-only).

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>