## Context
Related to Explorer/Notebooks - this PR adds the functionality to add /
remove cells
Separately - am thinking we can shift a lot of the "cell update" logic
into notebook-state, mainly so that each UI component doesn't need to be
aware of the notebook's `cells` but just its own cell.
I'll do it separately though to prevent bloating this PR, already left
comments where i think can be refactored
<img width="250" alt="image"
src="https://github.com/user-attachments/assets/91aae5f9-07b9-4327-8e21-8e9d71098aa6"
/>
<img width="250" alt="image"
src="https://github.com/user-attachments/assets/677de208-5c52-4155-90c6-bc0879070693"
/>
<img width="250" alt="image"
src="https://github.com/user-attachments/assets/17c3dae7-87bc-40bb-bfa0-15df101006ce"
/>
<img width="1091" height="262" alt="image"
src="https://github.com/user-attachments/assets/a09c3d23-696a-4b55-a1f8-4602e56a60dc"
/>
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
## New Features
- Add query and Markdown cells directly within notebooks.
- Move, remove, and reorder cells using drag-and-drop or cell controls.
- Add cells from empty states and notebook toolbar actions.
- Edit Markdown cells, mark edits as complete, and see placeholders for
empty content.
## Improvements
- Cell controls and hover interactions are more consistent and
responsive.
- Moving cells is disabled at the top or bottom of a notebook.
- Sample cells now use standardized content and formatting.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
