Refactor heatmap tab to use uploaded gene data

[j-andrews7]

Summary

• update the Heatmaps tab UI and help text to focus on uploaded gene summary data and new display controls
• build the gene heatmap from user datasets with selectable value terms, scaling, clustering toggles, and click details
• adjust tab enablement and download handlers to work with the new user-data-driven heatmap

Testing

• Not run (Rscript not available in container)

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Codex Task