Render scale bar as deck.gl overlay

[cornhundred]

Summary

• render the landscape scale bar with deck.gl polygon and text layers that compute pixel-to-micron dimensions
• update IST view handling to include the scale bar layers and refresh them from debounced view-state changes
• add scale bar layer management to SST and H&E views so the overlay toggles with zooming and reappears after interactions

Testing

• npm run lint:js

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https://chatgpt.com/codex/tasks/task_b_68faafe04330832aae4ac338b980eb42