Link POI popups to offline Wikipedia via kiwix-serve

lib/maptool/kiwix.rb: a small reachability check for an external
kiwix-serve instance. GET /api/kiwix reports it (same optional-
integration pattern as offline-tiles/terrain-tiles); the frontend
adds a "Nearby on Wikipedia" link to a point's popup pointing straight
at kiwix-serve's own geo search (GET /search?latitude=&longitude=
&distance=, confirmed from libkiwix's actual source rather than
guessed) — linked to directly, not proxied/embedded, since it already
has a full search UI of its own.

docker-compose.yaml: new `kiwix` service (ghcr.io/kiwix/kiwix-serve)
on the default network, KIWIX_URL wired to reach it by service name.
The ZIM itself isn't part of the repo (large, self-contained archive)
— data/kiwix/ gitignored same as the *.pmtiles files, setup steps in
docs/development.adoc.

No kiwix-serve/ZIM available here to test end-to-end — verified the
app side against a stand-in local HTTP server (GET /api/kiwix flips
available correctly) and confirmed the generated popup link matches
the real geo-search query shape. docker-compose.yaml validated with
`docker-compose config`. Full test suite: 118 runs, 0 failures.

public/js/gpx.js

@@ -702,7 +702,7 @@ export class GpxPanelControl {
       const editable = feature.properties.category === "Note" && !!feature.properties.id && isLoggedIn();
       const popup = new Popup({ closeButton: true, maxWidth: "260px" })
         .setLngLat(event.lngLat)
-        .setHTML(popupHtml(feature.properties, { editable }))
+        .setHTML(popupHtml(feature.properties, { editable, lngLat: event.lngLat }))
         .addTo(this._map);
       if (editable) this._wireNoteActions(popup, path, feature);
     });
@@ -754,7 +754,7 @@ export class GpxPanelControl {
       const bodyInput = content.querySelector(".gpx-popup__edit-body");
       titleInput.focus();
       content.querySelector(".gpx-popup__edit-cancel").addEventListener("click", () => {
-        content.innerHTML = popupHtml(feature.properties, { editable: true });
+        content.innerHTML = popupHtml(feature.properties, { editable: true, lngLat: popup.getLngLat() });
         this._wireNoteActions(popup, path, feature);
       });
       content.querySelector(".gpx-popup__edit-save").addEventListener("click", async () => {
@@ -1077,13 +1077,26 @@ function gpxFileUrl(path) {
   return `/api/gpx/file/${path.split("/").map(encodeURIComponent).join("/")}`;
 }
 
+// Set once from map.js after its one-time GET /api/kiwix check (same
+// fetch-once-at-load story as the offline-tiles/terrain-tiles checks) —
+// module-level rather than threaded through every popupHtml call, since
+// it's the same answer for the whole page's lifetime.
+let kiwixConfig = { available: false, url: "" };
+
+export function setKiwixConfig(config) {
+  kiwixConfig = config;
+}
+
 // properties come straight from an uploaded GPX file — untrusted input —
 // so every value is escaped before landing in the popup's HTML.
 // editable (only ever true for a "Note" feature the viewer is logged in
 // for — see _registerPointInteractivity) adds Edit/Delete buttons; the
 // click handling for them lives in _wireNoteActions, not here, since this
-// only builds markup, it has no map/store access of its own.
-function popupHtml({ name, description, time, category, photos, icon }, { editable = false } = {}) {
+// only builds markup, it has no map/store access of its own. lngLat (the
+// popup's own anchor point) drives the "Nearby on Wikipedia" link — kiwix-
+// serve's own geo search, linked to directly rather than proxied/embedded
+// (it already has a full search UI of its own, see docs/development.adoc).
+function popupHtml({ name, description, time, category, photos, icon }, { editable = false, lngLat = null } = {}) {
   const parts = [];
   if (name) {
     const glyph = icon && POI_ICONS[icon] ? `<span class="gpx-popup__header-icon">${POI_ICONS[icon]}</span>` : "";
@@ -1104,6 +1117,10 @@ function popupHtml({ name, description, time, category, photos, icon }, { editab
       parts.push(`<p class="gpx-popup__time">${escapeHtml(parsed.toLocaleString())}</p>`);
     }
   }
+  if (kiwixConfig.available && lngLat) {
+    const wikiUrl = `${kiwixConfig.url}/search?pattern=&latitude=${lngLat.lat}&longitude=${lngLat.lng}&distance=2000`;
+    parts.push(`<p class="gpx-popup__wiki"><a href="${escapeHtml(wikiUrl)}" target="_blank" rel="noopener noreferrer">Nearby on Wikipedia →</a></p>`);
+  }
   if (editable) {
     parts.push(
       `<div class="gpx-popup__note-actions"><button type="button" class="gpx-popup__edit">Edit</button><button type="button" class="gpx-popup__delete">Delete</button></div>`,