Replace the per-popup "Nearby on Wikipedia" link with a live Wikipedia layer

The old link only ever surfaced one nearby article at a time, buried
in whatever POI's popup you happened to open, and only for uploaded
files with a category. Replaced it with a standalone "Wikipedia"
toggle under the renamed "Tracks & Points" panel: enabling it adds a
live map layer that refetches GET /api/kiwix/nearby on every pan/zoom
and plots every nearby article as its own point.

kiwix-serve's own geo search never echoes back a result's coordinates
(confirmed live against the real production ZIM, both its HTML and
?format=xml output) — it only filters by distance from a point. So
Maptool::Kiwix.nearby fetches each matching article's own page too and
reads its <meta name="geo.position"> tag, which every geotagged
Wikipedia article carries. Tried parallelizing those per-article
fetches with threads first; measured no improvement against the real
ZIM (kiwix-serve appears to serialize requests regardless), so it
stayed sequential — ~1.6s for a 30-result page, fine for a
refetch-on-pan-end layer.

Clicking a point opens a small popup linking to the article (proxied
through the existing /kiwix/* passthrough). New WIKI_ICON (NPS's
bookstore-22.svg) marks these, kept out of POI_ICONS since that set
doubles as notes.js's icon-picker option list and this fixed-glyph
layer has no per-feature category to key off of.

app.rb

@@ -238,6 +238,19 @@ module Maptool
       json available: reachable, url: (reachable ? "/kiwix" : "")
     end
 
+    # Backs the "Wikipedia" map layer (public/js/gpx.js) — replaces the old
+    # per-popup "Nearby on Wikipedia" link, which only ever pointed at one
+    # POI at a time. Read-only and login-free, same as the tree/plugin
+    # listing routes below: viewing the map needs no account, only writing
+    # to it does.
+    get "/api/kiwix/nearby" do
+      halt 404 if KIWIX_URL.empty?
+      bbox = %w[south west north east].map { |key| params[key].to_s }
+      halt 400, json(error: "south, west, north, east are required") if bbox.any?(&:empty?)
+
+      json Kiwix.nearby(KIWIX_URL, bbox.map(&:to_f))
+    end
+
     # Proxies kiwix-serve's own search UI and content pages through this
     # app instead of linking the browser to it directly (see KIWIX_URL) —
     # kiwix-serve is started with --urlRootLocation=/kiwix