Display "Click to add a cell" button as a part of notebook, not background

### Problem

Currently "Click to add a cell" is rendered as a part of background / outer notebook area (grey in the JupyterLab light theme) under the drop shadow from the notebook / inner notebook area (white in the JupyterLab light theme).

To me it seems that the way how button is rendered in JupyterLab, as a part of notebook / inner notebook area (white in the JupyterLab light theme) is easier to read / parse. As an added bonus, this would make design language used in lab and notebook more consistent.

Notebook:

JupyterLab:

### Proposed Solution

Render "Click to add a cell" button as a part of notebook / inner notebook area (white in the JupyterLab light theme).

In particular white space at the bottom of inner notebook area could be made larger to accommodate the button when hovering in a way that there would be no need to add / "pop-in" any artificial white space.

It would make sense to add the same amount of white space to at the top of the notebook for visual balance.

Start by reviewing the proposed solution and the linked pull request #7587, then inspect the notebook rendering entry point used for the “Click to add a cell” button. Done means the button appears within the inner notebook area, with balanced top and bottom whitespace and no artificial pop-in space.

Written by the indexing model from the issue text.