Demand data for predict impact mode

- [ ] Verify the OD data we're using now is fine, or see what else is available (partly blocked on NPW)

Debug demand UI:

- [x] Mimic the "commuter patterns" UI from A/B Street. Use a heatmap per zone to count total trips from or to. Once focused on an area, re-scale the heatmap.
- [ ] Also consider using count symbols
- [ ] Link to this from predict impact mode (with a better back button)

The issue involves adding a demand data visualization UI, mimicking the 'commuter patterns' heatmap from A/B Street. Start by examining the existing UI code for predict impact mode and the referenced A/B Street heatmap implementation. Look for files related to map rendering, zone data handling, and trip counting. The work requires integrating a new visualization, rescaling it when focused, and linking it from predict impact mode with navigation.

Written by the indexing model from the issue text.