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).

docs/third-party-notices.adoc

@@ -21,6 +21,10 @@ BSD-3-Clause licensed.
 | protomaps-themes-base
 | 4.5.0
 | https://github.com/protomaps/basemaps
+
+| maplibre-contour (Overlays panel's "Contour lines", `public/js/contour.js`)
+| 0.1.0
+| https://github.com/onthegomap/maplibre-contour
 |===
 
 Also vendored under `public/vendor/`, but not BSD-3-Clause like the