Add a hillshade/terrain-shading toggle to the map

Sidebar toggle (new HillshadeControl) adds a raster-dem hillshade
layer: public Terrarium tiles when online, or a local
data/terrain.pmtiles when present (mirrors offline.pmtiles' optional-
local-file pattern, new /api/terrain-tiles + /tiles/terrain.pmtiles
endpoints). No offline terrain data is bundled, so the toggle is
online-only until one is generated — see docs/development.adoc for
how.

public/js/icons.js

@@ -7,6 +7,12 @@ export const LAYERS_ICON = `
   <path d="M2 14l10 6 10-6"/>
 </svg>`;
 
+export const HILLSHADE_ICON = `
+<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linejoin="round" stroke-linecap="round">
+  <path d="M2 18 9 7l3.5 5.5L15 9l7 9z"/>
+  <path d="M2 18h20"/>
+</svg>`;
+
 export const GPX_ICON = `
 <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linecap="round" stroke-linejoin="round">
   <circle cx="5" cy="6" r="2"/>

public/js/map.js

@@ -11,7 +11,7 @@ import { LiveTrackPanelControl } from "/js/livetrack.js";
 import { TrackProfilePanel } from "/js/track_profile.js";
 import { MeasureControl } from "/js/measure.js";
 import { LocateControl } from "/js/locate.js";
-import { LAYERS_ICON } from "/js/icons.js";
+import { LAYERS_ICON, HILLSHADE_ICON } from "/js/icons.js";
 import { notifySidePanelOpen, onlyOneSidePanelOpen } from "/js/panels.js";
 import HomeButton from "/vendor/npmap-home-button/home-button.esm.js";
 import OverviewMap from "/vendor/npmap-overview/overview.esm.js";
@@ -305,6 +305,73 @@ class LayerToggleControl {
   }
 }
 
+const HILLSHADE_ID = "hillshade-dem";
+// Public, unauthenticated, no API key — the same open (SRTM/ArcticDEM-derived)
+// elevation tiles OpenStreetMap's own hillshade layer and many other free
+// tools use. Only reachable online; while the offline basemap is active this
+// is skipped in favor of a local terrain.pmtiles if one's been generated
+// (see docs/development.adoc), same optional-local-file story as
+// offline.pmtiles itself.
+const ONLINE_TERRAIN_TILES = ["https://s3.amazonaws.com/elevation-tiles-prod/terrarium/{z}/{x}/{y}.png"];
+
+// A simple on/off sidebar toggle, not a panel — there's nothing to choose,
+// just whether the shading shows.
+class HillshadeControl {
+  constructor(map) {
+    this._map = map;
+    this._active = false;
+    this._offlineAvailable = false;
+    // setStyle() (switching Online/Offline) tears down all custom
+    // sources/layers — put hillshade back if it was on.
+    map.on("style.load", () => {
+      if (this._active) this._addLayer();
+    });
+  }
+
+  mount(sidebar) {
+    this._container = document.createElement("div");
+    this._container.className = "sidebar-item";
+    this._container.innerHTML = `<button type="button" class="sidebar-icon" title="Terrain shading" aria-label="Terrain shading" aria-pressed="false">${HILLSHADE_ICON}</button>`;
+    this._button = this._container.querySelector(".sidebar-icon");
+    this._button.addEventListener("click", () => this._toggle());
+    sidebar.appendChild(this._container);
+  }
+
+  setOfflineAvailable(available) {
+    this._offlineAvailable = available;
+  }
+
+  _toggle() {
+    this._active = !this._active;
+    this._button.classList.toggle("active", this._active);
+    this._button.setAttribute("aria-pressed", String(this._active));
+    if (this._active) this._addLayer();
+    else this._removeLayer();
+  }
+
+  _addLayer() {
+    if (this._map.getSource(HILLSHADE_ID)) return;
+
+    const source = this._offlineAvailable
+      ? { type: "raster-dem", url: `pmtiles://${window.location.origin}/tiles/terrain.pmtiles`, encoding: "terrarium" }
+      : {
+          type: "raster-dem",
+          tiles: ONLINE_TERRAIN_TILES,
+          tileSize: 256,
+          encoding: "terrarium",
+          maxzoom: 15,
+          attribution: "Terrain tiles: Mapzen, SRTM, ArcticDEM",
+        };
+    this._map.addSource(HILLSHADE_ID, source);
+    this._map.addLayer({ id: HILLSHADE_ID, type: "hillshade", source: HILLSHADE_ID, paint: { "hillshade-exaggeration": 0.4 } });
+  }
+
+  _removeLayer() {
+    if (this._map.getLayer(HILLSHADE_ID)) this._map.removeLayer(HILLSHADE_ID);
+    if (this._map.getSource(HILLSHADE_ID)) this._map.removeSource(HILLSHADE_ID);
+  }
+}
+
 // A small map-corner button (grouped with Home/zoom, see below) that's a
 // second entry point to the same panel LayerToggleControl already manages
 // in the page's own sidebar — matching the National Park Service's own
@@ -454,6 +521,8 @@ profilePanel.mount();
 
 sidebar.insertAdjacentHTML("beforeend", `<div class="sidebar-group-label" title="Explorer"></div>`);
 new GpxPanelControl(map, profilePanel).mount(sidebar);
+const hillshadeControl = new HillshadeControl(map);
+hillshadeControl.mount(sidebar);
 
 sidebar.insertAdjacentHTML("beforeend", `<div class="sidebar-group-label" title="Offline hub"></div>`);
 layerToggle.mount(sidebar);
@@ -501,6 +570,11 @@ fetch("/api/offline-tiles")
     // failing, not evidence offline tiles are actually unavailable.
   });
 
+fetch("/api/terrain-tiles")
+  .then((response) => response.json())
+  .then(({ available }) => hillshadeControl.setOfflineAvailable(available))
+  .catch(() => {});
+
 // Alt+click anywhere shows every rendered feature at that point — base map
 // (OSM tags via OpenFreeMap/protomaps), GPX/FIT layers, radar, all of it —
 // with its raw properties, inspired by NPMap4's infoboxControl (a generic