Impact Mode: Refine the visualisation

Besides just studying the impact on shortcuts within one neighbourhood boundary, the user can see how traffic changes across roads in the whole map. This works by finding the best route before and after changes for every origin/destination "OD" pairs, then counting routes per road.

Features:

- [x] Demand model mvp: random OD pairs
- [x] Demand model v2: Use same NPW data for Scotland. Need to detect country and decide how to clip that data. For those special areas, prebake maps with everything bundled in? Or prebake clipped OD datasets that match map names?
- [x] Just do the calculation naively
- [x] Display relative results
- [ ] Experiment with showing absolute vs relative counts
- [ ] Let user change the main road penalty
- [ ] Change default main road penalty to > 1.0 ?
- [x] Click one road to see paths through it that changed
- [ ] Consider CSV export of the data (being careful about communicating the absolute vs relative change)

Not important, just a stray idea: Consider some kind of differential "spider map"; see https://github.com/Urban-Analytics-Technology-Platform/od2net/issues/35

El issue describe la mejora de la visualización de «Impact Mode» para mostrar los cambios del tráfico en todo el mapa. Empieza examinando el código existente del modelo de demanda y del cálculo de rutas, probablemente en módulos que gestionan OD pairs y redes de carreteras. Busca la lógica de visualización actual para entender cómo se muestran los resultados relativos. Las tareas incluyen experimentar con conteos absolutos frente a relativos, ajustar la main road penalty e implementar una funcionalidad click-to-see-paths. Comprueba si existen pruebas o ejemplos preexistentes relacionados con impact mode para ver el comportamiento actual.

Escrito por el modelo de indexación a partir del texto del issue.