improve: bring AI chat style to "generate cell with AI" area

[m-rgba]

📝 Summary

Before	After

Looking to make the "Generate cell with AI" input area consistent with the AI chat sidebar.

• Brings over "Add context" and "Attach file" buttons to the "Generate cell with AI" area.
• Moves instruction text into placeholder (adds a min-height to the editor to avoid banding when the user starts typing).
• Shifts the model selector and language to the left to have consistent placement.
• Updated the AI sparkles and close button to be pinned as the top row.

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Project	Deployment	Review	Updated (UTC)
marimo-docs	Ready	Preview, Comment	Jan 5, 2026 8:49pm

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