Group federal subjects in statistics

[Ainali]

The feature

In the statistics, federal subjects are currently shown on the same level as countries they are in. It would be nicer if they could be grouped somehow "under" the country.

Why?

This would make it clearer that they are on a different level, and how rich the data in such a country is.

Examples and inspiration

Supersedes #356

[noveshk7]

Grouping federal subjects under their respective countries would enhance clarity and better represent the data hierarchy. This can be achieved through indentation or nesting, expandable sections to toggle visibility, separate sections under country headings, or using subtle UI elements like colors, icons, or dividers to distinguish levels. These improvements would make it easier to understand the structure and richness of data within each country while keeping the interface visually organized and intuitive.