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.

TODO.adoc

@@ -14,7 +14,7 @@ independence from connectivity.
 * [ ] Contour lines from OSM for hikes and mountain passes
 * [x] Overpass/OSM scripts to extract POIs (drinking water, dump stations, LPG, public toilets, laundromats, shops, springs) — `rake overpass:pois`, see docs/development.adoc; writes a `.geojson` to drop in `data/gpx/` or upload, not called by the running app itself
 * [ ] Import Park4Night / iOverlander / Freecampsites data
-* [ ] Kiwix/ZIM offline Wikipedia linked to POI GPS coordinates
+* [x] Kiwix/ZIM offline Wikipedia linked to POI GPS coordinates — a "Nearby on Wikipedia" popup link straight to kiwix-serve's own geo search (`GET /search?latitude=&longitude=&distance=`, confirmed from its actual source, `libkiwix/internalServer.cpp`), not proxied/embedded — it already has a full search UI. `KIWIX_URL` + `GET /api/kiwix` reachability check, same optional-integration pattern as offline.pmtiles/terrain.pmtiles; `docker-compose.yaml` now runs a `kiwix` service, ZIM not included (see docs/development.adoc for where to get one). Not live-tested against a real ZIM (none available here) — verified via a stand-in local HTTP server + real popup-link generation
 * [x] Cache weather forecast when temporarily connected — Open-Meteo 3-day forecast in the Radar flyout, last successful fetch cached in localStorage as a fallback when offline
 ** [x] Cache the radar tile *images* too — `public/sw.js`, network-falling-back-to-cache for RainViewer tile requests, registered from layout.erb