Add a zoom-level readout to the map control stack

Matches the small numbered square NPS keeps next to its own zoom
buttons — a passive display, not a button, so it's plain markup with
no click behavior.

docs/architecture.adoc

@@ -145,8 +145,11 @@ app's default view, not the visitor's own location — that's
 
 That Home button is the top of a small top-left stack — Home, zoom
 in/out (`NavigationControl`, `showCompass: false` since this map never
-rotates), locate-me (`GeolocateControl`), and a "Base map" button —
+rotates), a passive zoom-level readout (`ZoomLevelControl`, a plain div
-matching the National Park Service's own map control layout (grouped
+updated on the map's `zoom` event — not a button, since it has no
+click behavior of its own), locate-me (`GeolocateControl`), and a
+"Base map" button — matching the National Park Service's own map
+control layout (grouped
 together on the map itself, rather than split across corners or
 between the map and a side panel). The last of those,
 `MapLayersButton` (`public/js/map.js`), is a second entry point into

public/css/style.css

@@ -472,6 +472,18 @@ html, body {
   word-break: break-word;
 }
 
+.zoom-level-ctrl__value {
+  width: 29px;
+  height: 29px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  font-size: 12px;
+  font-weight: 600;
+  color: #333;
+  user-select: none;
+}
+
 .gpx-popup__time {
   color: #888;
   font-size: 12px;

public/js/map.js

@@ -182,6 +182,39 @@ class MapLayersButton {
   }
 }
 
+// A passive zoom-level readout, sitting in the same stack as the buttons
+// around it — the National Park Service's own control layout has one (the
+// small numbered square next to their zoom buttons). Not a button: no
+// click behavior, just a live display, so it's a plain div rather than a
+// <button> (no hover/focus affordance implying it does something).
+class ZoomLevelControl {
+  onAdd(map) {
+    this._map = map;
+    this._container = document.createElement("div");
+    this._container.className = "maplibregl-ctrl maplibregl-ctrl-group zoom-level-ctrl";
+    this._label = document.createElement("div");
+    this._label.className = "zoom-level-ctrl__value";
+    this._container.appendChild(this._label);
+
+    this._update = () => {
+      this._label.textContent = Math.round(map.getZoom());
+    };
+    this._update();
+    map.on("zoom", this._update);
+
+    return this._container;
+  }
+
+  onRemove() {
+    this._map.off("zoom", this._update);
+    this._container.remove();
+  }
+
+  getDefaultPosition() {
+    return "top-left";
+  }
+}
+
 const map = new MaplibreMap({
   container: "map",
   style: ONLINE_STYLE,
@@ -195,10 +228,10 @@ const map = new MaplibreMap({
   hash: "map",
 });
 
-// A compact top-left stack — Home, zoom, locate-me, base map — matching
+// A compact top-left stack — Home, zoom, zoom-level readout, locate-me,
-// the National Park Service's own map control layout, all grouped
+// base map — matching the National Park Service's own map control layout,
-// together rather than split across corners. Added in this order since
+// all grouped together rather than split across corners. Added in this
-// MapLibre stacks same-position controls in add order.
+// order since MapLibre stacks same-position controls in add order.
 //
 // homePosition/zoom pinned to the app's own default rather than left to the
 // button's own fallback (current center/zoom at construction) — now that
@@ -211,6 +244,7 @@ map.addControl(new HomeButton({ homePosition: CENTER, zoom: ZOOM }), "top-left")
 // showCompass: false — this map never rotates, so a bearing-reset button
 // (NavigationControl's default third button) would just be dead weight.
 map.addControl(new NavigationControl({ showCompass: false }), "top-left");
+map.addControl(new ZoomLevelControl(), "top-left");
 map.addControl(
   new GeolocateControl({
     positionOptions: { enableHighAccuracy: true },