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/overlays.js

@@ -4,6 +4,7 @@ import { notifySidePanelOpen, onlyOneSidePanelOpen } from "/js/panels.js";
 import { svgToImageData, badgeImageId, escapeHtml } from "/js/gpx.js";
 import { RadarControl } from "/js/radar.js";
 import { ShadowControl } from "/js/shadow.js";
+import { ContourControl } from "/js/contour.js";
 
 const ICON_IMAGE_SIZE = 48; // rasterized once at this size, rendered much smaller by icon-size below
 const ICON_SIZE = 0.3; // 48 * 0.3 = ~14px, sized to sit inside the marker badge below
@@ -125,6 +126,7 @@ export class OverlaysControl {
     this._hillshade = new HillshadeControl(map);
     this._radar = new RadarControl(map);
     this._shadow = new ShadowControl(map);
+    this._contour = new ContourControl(map);
     this._wikiEnabled = false; // the checkbox's own state
     this._wikiMoveHandler = null; // the moveend listener driving its live refetch, while enabled
     this._wikiRequestSeq = 0; // ignores an in-flight refetch a newer one has already superseded
@@ -157,12 +159,14 @@ export class OverlaysControl {
       <li class="overlays-panel__item"></li>
       <li class="overlays-panel__item overlays-panel__item--expandable"></li>
       <li class="overlays-panel__item overlays-panel__item--expandable"></li>
+      <li class="overlays-panel__item"></li>
     `;
-    const [hillshadeItem, wikiItem, radarItem, shadowItem] = list.children;
+    const [hillshadeItem, wikiItem, radarItem, shadowItem, contourItem] = list.children;
     this._hillshade.mount(hillshadeItem);
     this._mountWiki(wikiItem);
     this._radar.mount(radarItem);
     this._shadow.mount(shadowItem);
+    this._contour.mount(contourItem);
 
     onlyOneSidePanelOpen(this._panel, () => this._icon.classList.remove("active"));