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.

test/test_app.rb

@@ -23,6 +23,11 @@ class AppTest < Minitest::Test
     assert_equal({ "available" => false }, json_body(response))
   end
 
+  def test_kiwix_reports_unavailable_when_no_url_configured
+    response = request.get("/api/kiwix")
+    assert_equal({ "available" => false, "url" => "" }, json_body(response))
+  end
+
   def test_gnss_position_unavailable_with_no_fix
     response = request.get("/api/gnss/position", **logged_in_env)
     assert_equal({ "available" => false }, json_body(response))