Display existing notebooks on the new page

[pearmini]

@jackbdu Jack mentioned that he wasn’t aware the notebook was saved after navigating away from the new page. To address this, I think it would be helpful to give users a hint that there’s a Notebooks page.

Taking inspiration from Github Gist, we can display a few existing notebooks directly on the new page as a subtle cue:

Here’s how the updated new page looks:

And when there are no notebooks, it appears like this:

[cloudflare-workers-and-pages]

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[Copilot]

Pull Request Overview

This PR adds a section to the editor page that displays existing notebooks when creating a new notebook, helping users discover the notebooks page and understand that their work is being saved.

• Added a header section that shows up to 4 recent notebooks when creating a new notebook
• Displays a tagline when no notebooks exist
• Added responsive design with different layouts for desktop and mobile

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