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/map.js

@@ -5,7 +5,7 @@ import {
   addProtocol,
 } from "/vendor/maplibre-gl/maplibre-gl.mjs";
 import { RadarControl } from "/js/radar.js";
-import { GpxPanelControl } from "/js/gpx.js";
+import { GpxPanelControl, setKiwixConfig } from "/js/gpx.js";
 import { GnssControl } from "/js/gnss.js";
 import { LiveTrackPanelControl } from "/js/livetrack.js";
 import { TrackProfilePanel } from "/js/track_profile.js";
@@ -609,6 +609,11 @@ fetch("/api/terrain-tiles")
   .then(({ available }) => hillshadeControl.setOfflineAvailable(available))
   .catch(() => {});
 
+fetch("/api/kiwix")
+  .then((response) => response.json())
+  .then(setKiwixConfig)
+  .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