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.

app.rb

@@ -10,6 +10,7 @@ require_relative "lib/maptool/gpx_store"
 require_relative "lib/maptool/gpsd_client"
 require_relative "lib/maptool/livetrack_store"
 require_relative "lib/maptool/livetrack_client"
+require_relative "lib/maptool/kiwix"
 
 module Maptool
   class App < Sinatra::Base
@@ -69,6 +70,12 @@ module Maptool
     # OAuth2 secrets: fine outside production, nothing works without it set.
     LIVETRACK_WEBHOOK_SECRET = ENV.fetch("LIVETRACK_WEBHOOK_SECRET", "")
 
+    # An external kiwix-serve instance (see docs/development.adoc), not
+    # something this app runs itself — blank disables the "Nearby on
+    # Wikipedia" popup link the same way the other optional-integration
+    # blanks above do.
+    KIWIX_URL = ENV.fetch("KIWIX_URL", "")
+
     # Login delegates to the Gitea instance the user already has an account
     # on, rather than maintaining a local username/password — see
     # docs/architecture.adoc. Only users listed in ALLOWED_GITEA_USERS may
@@ -199,6 +206,13 @@ module Maptool
       send_file TERRAIN_PMTILES_PATH, type: "application/octet-stream"
     end
 
+    # Pinged once per page load (public/js/map.js) — url is handed back so
+    # the frontend can build the geo-search link itself; this server never
+    # proxies to kiwix-serve, just says whether it's there.
+    get "/api/kiwix" do
+      json available: Kiwix.reachable?(KIWIX_URL), url: KIWIX_URL
+    end
+
     get "/api/gpx/tree" do
       json GPX_STORE.tree
     end