Modify color scale and classification

When coloring by a variable, the color scale and classification are hard-coded.

I think in descending priority it would be good to add:

1. Color scale inversion (as a high price can be positive or negative)
2. Class number (how many classes you want)
3. Custom colors for each class
4. Custom class breaks and/or automatic ones (Jenks etc.)

For inspiration: QGIS has the most complete class and color editor I know of. Reproducing the full editor on the web might be overkill of course, but I think it's worth a look.

Start by locating the existing color-scale and classification controls in the visualization UI, then compare their current behavior with the prioritized requests in this issue. Define a focused scope—beginning with scale inversion—and document how class counts, colors, and breaks should be configured before implementation; done means the selected controls change the rendered coloring as intended.

Written by the indexing model from the issue text.