Add contour lines to the Overlays panel via maplibre-contour

5th Overlays item, generated client-side from the same online Terrarium
DEM tiles Terrain shading already fetches (no offline fallback — the
plugin's DemSource has no public hook to route through the pmtiles://
protocol the offline basemap uses, so this quietly renders nothing
without a network, same as Radar/the Wikipedia layer).

A plain vector source/layer under MapLibre's own tile lifecycle, no
manual refetch-on-move code needed. Thresholds are the plugin's own
generic starting point, not yet tuned against real terrain. Labels
skipped for now (font-name matching across the online/offline styles'
own glyph sets adds real risk for a nice-to-have, not the actual ask).

public/js/radar.js

@@ -357,7 +357,9 @@ function readCachedForecast() {
   }
 }
 
-function firstSymbolLayerId(map) {
+// Exported: contour.js's own contour-line layers want the same
+// "sit above base fill/roads, below place labels" placement.
+export function firstSymbolLayerId(map) {
   const layers = map.getStyle()?.layers || [];
   const symbolLayer = layers.find((layer) => layer.type === "symbol");
   return symbolLayer ? symbolLayer.id : undefined;