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.

docker-compose.yaml

@@ -21,6 +21,13 @@ services:
       # Optional: blank disables the LiveTrack webhook endpoint rather than
       # failing to start, since it's not needed for the app's core function.
       LIVETRACK_WEBHOOK_SECRET: ${LIVETRACK_WEBHOOK_SECRET:-}
+      # The kiwix service below, reached by its compose service name (both
+      # are on the same default compose network) — port matches its own
+      # --port=8080 in the command below. Not made conditional on the ZIM
+      # actually being present: GET /api/kiwix pings it and just reports
+      # unavailable if the container isn't up/reachable, same story as
+      # every other optional-local-file feature (offline.pmtiles, etc).
+      KIWIX_URL: "http://kiwix:8080"
     volumes:
       # Uploaded GPX files and the offline PMTiles extract — kept outside
       # the image so a rebuild/redeploy doesn't wipe them (see
@@ -43,3 +50,19 @@ services:
     restart: unless-stopped
     network_mode: "service:maptool"
     command: ["ruby", "bin/gnss_simulator.rb"]
+
+  # Serves an offline Wikipedia ZIM for the popup's "Nearby on Wikipedia"
+  # link (see docs/development.adoc for where to get one and drop it in —
+  # same "not part of the repo, place it under ./data locally" story as
+  # offline.pmtiles/terrain.pmtiles). On the default compose network (not
+  # network_mode: "service:maptool" — unlike gnss-simulator this doesn't
+  # need a hardcoded localhost port, KIWIX_URL above reaches it by this
+  # service's own name), so maptool and kiwix stay independent containers.
+  # Keeps restarting (harmlessly) if data/kiwix/wikipedia.zim isn't there
+  # yet — GET /api/kiwix just reports unavailable until it comes up.
+  kiwix:
+    image: ghcr.io/kiwix/kiwix-serve:latest
+    restart: unless-stopped
+    command: ["--port=8080", "/data/wikipedia.zim"]
+    volumes:
+      - ./data/kiwix:/data