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/css/style.css

@@ -373,6 +373,21 @@ html, body {
   color: #666;
 }
 
+.gpx-panel__wiki {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  margin: 0 12px 8px;
+  padding-bottom: 8px;
+  border-bottom: 1px solid #e5e5e5;
+  font-size: 13px;
+  cursor: pointer;
+}
+
+.gpx-panel__wiki[hidden] {
+  display: none;
+}
+
 .gpx-tree {
   /* flex:1 (not just overflow-y) so the tree fills the rest of the panel
      even when short/empty — otherwise the blank space below it wouldn't