Add dataset rotation and cell name prefix support

[cornhundred]

Summary

• allow the widget to accept multiple base URLs and expose a cell_name_prefix flag for trimming prefixed cell IDs
• add a dataset dropdown and UI refresh hooks so deck.gl layers can rotate through datasets without rebuilding the visualization
• normalize cell identifiers before loading landscape files and reuse controls when datasets change

Testing

• Not run (not requested)

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Codex Task