Add diverging heatmap visualization to embedding explorer

[DenisValeev]

Summary

• add a visualization selector to the embedding explorer so visitors can pick between heatmap and raw byte renderings
• render a green/yellow/red diverging heatmap from float vectors while retaining the RGB byte fallback and exposing the active mode in metadata
• refresh comparison copy and captions to reflect the new visualization control

Testing

• npm test (fails: Playwright browsers missing on host environment)

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https://chatgpt.com/codex/tasks/task_e_68d5383c94a08328821416bad9f28e62