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).

views/layout.erb

@@ -21,6 +21,7 @@
   </script>
   <script src="/vendor/pmtiles/pmtiles.js"></script>
   <script src="/vendor/protomaps-themes-base/protomaps-themes-base.js"></script>
+  <script src="/vendor/maplibre-contour/maplibre-contour.min.js"></script>
   <%# Vendored libs above don't change between deploys, so they're left
       unversioned; app.js's own modules do, and map.js statically imports
       them by bare path — an import map remaps those paths to a versioned
@@ -31,6 +32,7 @@
     {
       "imports": {
         "/js/auth.js": "/js/auth.js?v=<%= ASSET_VERSION %>",
+        "/js/contour.js": "/js/contour.js?v=<%= ASSET_VERSION %>",
         "/js/dem.js": "/js/dem.js?v=<%= ASSET_VERSION %>",
         "/js/gnss.js": "/js/gnss.js?v=<%= ASSET_VERSION %>",
         "/js/gpx.js": "/js/gpx.js?v=<%= ASSET_VERSION %>",