Add markdown cell support

Hello,

Thank you for releasing this awesome interface! Makes it so easy to use DuckDB.

Is it possible to add support for markdown cells? I like to annotate and organize my notebooks and this would really help. Thanks for considering.

The issue names no files, tests, or entry points; begin by locating the notebook cell UI and its rendering/editing flow. Define done as being able to create and use a markdown cell for annotating and organizing a notebook, with the expected behavior confirmed by the project’s existing checks.

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