Feat: Support multiple layers / geometries for same source

Objective

As a coordo user, I want to be able to define 2 layers for the same data source in the config.json

Context

In MapLibre, Sources and Layers are separated components that allow a user to render the same data in multiple manners.
https://maplibre.org/maplibre-gl-js/docs/examples/add-multiple-geometries-from-one-geojson-source/

In Data4Trees, what we want to do is the following :

For Seed :
Have one layer that displays icons and when clicked the popup with indicators is displayed. This layer would hide if the zoom become big.
When the zoom becomes big, another layer is made visible : this layer shows polygons filled with color, but the same popup as in the other layer is rendered when a polygon is clicked.

All4Trees:
Have one layer that displays icons and when clicked the popup with indicators is displayed. This layer would hide if the zoom become big.
When the zoom becomes big, another layer is made visible : this layer shows circles, but the same popup as in the other layer is rendered when a polygon is clicked.

Start with config.json and the MapLibre multiple-geometries example linked in the issue. Trace how Seed and All4Trees sources, layers, zoom visibility, and popups are currently configured. Done means each source supports icon and geometry layers with zoom-based switching and the same indicators popup for both.