Chore: Improve default layers of config.json

Currently we use basic layers with merkator projection :

  {"layers": [
    {
      "id": "satellite",
      "type": "xyzservices",
      "provider": "Esri.WorldImagery"
    },
    {
      "id": "villages",
      "type": "openmaptiles",
      "layer": "place",
      "filters": {
        "class": "village"
      }
    },
    {
      "id": "towns",
      "type": "openmaptiles",
      "layer": "place",
      "filters": {
        "class": "town"
      }
    },
    {
      "id": "boundaries",
      "type": "openmaptiles",
      "layer": "boundary",
      "filters": {
        "admin_level": 2
      },
      "style": {
        "paint": {
          "line-color": "hsl(248,7%,66%)"
        }
      }
    }]}

The idea of this PR is to use a non-merkator projection rto represent Africa with its real size. Also the goal would be to show a more detailed map with road and big cities, as it seems now only villages / regions are shown.

Start with config.json and trace how the default layers and projection are loaded. Review the current layer definitions for the satellite, place, and boundary entries, then determine which supported non-Mercator projection and map sources can provide roads and major cities. Done means Africa is displayed with a non-Mercator projection and the default map includes the requested detail.