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.

public/js/icons.js

@@ -353,6 +353,24 @@ Object.assign(POI_ICONS, {
   shop_convenience: STORE_ICON,
 });
 
+// src/bookstore-22.svg (NPS symbol library, same source/license as
+// POI_ICONS above) — used as the fixed marker glyph for the "Wikipedia"
+// map layer (gpx.js), not part of POI_ICONS itself: that set is keyed by
+// OSM/p4n2gpx category and doubles as the note icon-picker's option list
+// (notes.js), neither of which applies here — every point on this layer
+// gets the same one glyph, there's no per-feature category to key it by.
+export const WIKI_ICON = `
+<svg viewBox="0 0 22 22" fill="currentColor">
+  <path d="M16.6,20.7c-0.6,0-0.9-0.4-0.9-0.9c0-0.5,0.3-0.9,0.9-0.9c0.3,0,0.5-0.2,0.5-0.5
+	S16.9,18,16.6,18c-1.1,0-1.9,0.8-1.9,1.9c0,1,0.7,1.9,1.9,1.9c0.3,0,0.5-0.2,0.5-0.5S16.9,20.7,16.6,20.7L16.6,20.7z"/>
+  <path d="M5,18c0,0,0,0,0,0c0,3.1,0.5,4,2.7,4l8.9,0c0.3,0,0.5-0.2,0.5-0.5
+	c0-0.3-0.2-0.5-0.5-0.5H8c-1.5,0-2-0.5-2-3c0,0,0,0,0-0.1c0-0.3-0.2-0.5-0.5-0.5S5,17.7,5,18L5,18z"/>
+  <path d="M5,8c0-0.6,5-5,5-5h5h1c0.6,0,1,0.4,1,1v14c0,0.6-0.4,1-1,1H6c-0.6,0-1-0.4-1-1
+	V8z M9,4L6,18c0,0,10,0,10,0c0,0,0-14,0-14C16,4,9,4,9,4z"/>
+  <path d="M5,6.8c0-0.5,0.4-1.3,0.8-1.6l6.4-4.7C12.7,0.2,13,0.4,13,1v10
+	c0,0.6-0.3,1.7-0.7,2.1l-6.5,5.3C5.4,18.8,5,18.6,5,18V6.8z"/>
+</svg>`;
+
 export const LIVETRACK_ICON = `
 <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linecap="round" stroke-linejoin="round">
   <path d="M12 21s7-7.58 7-12a7 7 0 0 0-14 0c0 4.42 7 12 7 12z"/>