feat: support format_mapping for dataframe as table

[hodlen]

📝 Summary

Adds format_mapping support to mo.ui.dataframe so display formatting is applied in the dataframe UI preview/search payloads, plus docs and tests.

🔍 Description of Changes

• Threaded format_mapping through dataframe search serialization so formatted values appear in the UI.
• Added a regression test that asserts formatted values are returned in search results.
• Documented format_mapping usage in the interactive dataframes guide.

📋 Checklist

• I have read the contributor guidelines.
• For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on Discord, or the community discussions (Please provide a link if applicable).
• I have added tests for the changes made.
• I have run the code and verified that it works as expected.

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Project	Deployment	Review	Updated (UTC)
marimo-docs	Ready	Preview, Comment	Jan 8, 2026 5:44pm