feat(toolbar): insert cells after focused cell

[rgbkrk]

The toolbar's +Code and +Markdown buttons now insert new cells after the currently focused cell, instead of always prepending at the top. If no cell is focused, the new cell is inserted after the last cell (or at the top if the notebook is empty).

This matches the intuitive behavior of the inline "Add Cell" buttons that appear between cells.

Verification

• Focus a cell and click +Code — new cell appears below focused cell
• Focus a cell and click +Markdown — new cell appears below focused cell
• Click outside cells to unfocus, then click +Code — new cell appears at bottom
• Create a new notebook and click +Code — cell is added

PR submitted by @rgbkrk's agent, Quill

[Copilot]

Pull request overview

This PR modifies the notebook toolbar's +Code and +Markdown buttons to insert new cells contextually — after the currently focused cell — rather than always prepending at the top. When no cell is focused, cells are appended after the last existing cell (or at position 0 for empty notebooks). The addCell function in useAutomergeNotebook already accepted an optional afterCellId parameter, so the changes are limited to threading the focused cell ID and last cell ID to the toolbar component.

Changes:

• Extended NotebookToolbarProps with focusedCellId and lastCellId props, and updated onAddCell signature to accept an optional afterCellId.
• Updated toolbar button click handlers to pass focusedCellId ?? lastCellId as the insertion target.
• Threaded the new props from AppContent to NotebookToolbar, computing lastCellId from the cells array.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
apps/notebook/src/components/NotebookToolbar.tsx	Added focusedCellId and lastCellId props; updated +Code and +Markdown button handlers to pass the insertion position.
apps/notebook/src/App.tsx	Passes focusedCellId and computed lastCellId to the NotebookToolbar component.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

You can also share your feedback on Copilot code review. Take the survey.