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.

TODO.adoc

@@ -16,7 +16,8 @@ independence from connectivity.
 * [x] Import plugins: a pluggable way to add more data sources (Park4Night, iOverlander, Freecampsites, ...) without their scrapers necessarily living in this (public) repo — sidebar icon (logged in only) lists whatever's under `plugins/`, "Run for this area" uses the current map viewport as the bbox, no separate area-picker needed. Contract: a `plugin.json` manifest + a script that reads `--south`/`--west`/`--north`/`--east` and prints a GeoJSON `FeatureCollection` to stdout (`lib/maptool/plugins.rb`, `GET /api/plugins`, `POST /api/plugins/:name/run`) — output saved the same way an upload would be. `plugins/overpass-pois` ships as the reference example; a private/ToS-sensitive scraper just drops its own directory in locally and is never `git add`ed
 ** [x] Park4Night — private plugin (`data/plugins/p4n2gpx/`, outside this repo per the "ToS-sensitive scraper" note above); iOverlander/Freecampsites still open, same story
 ** [x] A private plugin's own dependencies now have somewhere to live too: `data/plugins/` is under the persistent `data/` bind mount (survives a redeploy, unlike `build/`), and `docs/deployment.adoc` documents the convention
-* [x] Kiwix/ZIM offline Wikipedia linked to POI GPS coordinates — a "Nearby on Wikipedia" popup link to kiwix-serve's own geo search (`GET /kiwix/search?latitude=&longitude=&distance=`), proxied through this app (`GET /kiwix/*` in app.rb) rather than a direct link to kiwix-serve — production sits behind a reverse proxy that only forwards to this app, so no address exists for a browser to reach kiwix-serve at directly; kiwix-serve runs with `--urlRootLocation=/kiwix` so its own emitted links already match. `KIWIX_URL` + `GET /api/kiwix` reachability check, same optional-integration pattern as offline.pmtiles/terrain.pmtiles; `docker-compose.yaml` now runs a `kiwix` service, ZIM not included (see docs/development.adoc for where to get one)
+* [x] Kiwix/ZIM offline Wikipedia linked to POI GPS coordinates — proxied through this app (`GET /kiwix/*` in app.rb) rather than a direct link to kiwix-serve — production sits behind a reverse proxy that only forwards to this app, so no address exists for a browser to reach kiwix-serve at directly; kiwix-serve runs with `--urlRootLocation=/kiwix` so its own emitted links already match. `KIWIX_URL` + `GET /api/kiwix` reachability check, same optional-integration pattern as offline.pmtiles/terrain.pmtiles; `docker-compose.yaml` now runs a `kiwix` service, ZIM not included (see docs/development.adoc for where to get one)
+** [x] Replaced the old per-popup "Nearby on Wikipedia" link (one POI at a time, easy to miss) with a standalone "Wikipedia" layer under Tracks & Points — a live `GET /api/kiwix/nearby` refetch on every pan/zoom, not a saved file. kiwix-serve's own geo search never echoes back a result's coordinates, so `Maptool::Kiwix.nearby` fetches each matching article's own page too and reads its `<meta name="geo.position">` tag — verified against the real production ZIM (~1.6s for a 30-result page; tried parallelizing those fetches with threads, no measured gain, kept sequential)
 * [x] Cache weather forecast when temporarily connected — Open-Meteo 3-day forecast in the Radar flyout, last successful fetch cached in localStorage as a fallback when offline
 ** [x] Cache the radar tile *images* too — `public/sw.js`, network-falling-back-to-cache for RainViewer tile requests, registered from layout.erb