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.

lib/maptool/kiwix.rb

@@ -0,0 +1,21 @@
+require "net/http"
+
+module Maptool
+  # A thin reachability check for an external kiwix-serve instance serving
+  # an offline Wikipedia ZIM (see docs/development.adoc for how to set one
+  # up) — the app itself never talks to it beyond this ping. Linking a POI
+  # popup to a geo search there (public/js/gpx.js) is a plain browser <a
+  # href> straight to kiwix-serve, not something this server proxies —
+  # kiwix-serve already has its own full search UI, no reason to rebuild it.
+  module Kiwix
+    def self.reachable?(url, open_timeout: 1, read_timeout: 2)
+      return false if url.to_s.strip.empty?
+
+      uri = URI(url)
+      Net::HTTP.start(uri.host, uri.port, open_timeout: open_timeout, read_timeout: read_timeout) { |http| http.head("/") }
+      true
+    rescue StandardError
+      false
+    end
+  end
+end