Move the Wikipedia layer into its own "Overlays" panel

Wikipedia doesn't belong inside "Tracks & Points" — that panel is for
files you add/remove/reorder, while Wikipedia is a live layer with
nothing to manage, just on/off. Gave it its own sidebar panel
(#overlays-panel, same shell as LiveTrackPanelControl) instead, sized
to hold more such checklist-only layers later without a redesign.

Moved all the wiki layer logic out of GpxPanelControl into a new
OverlaysControl (public/js/overlays.js). svgToImageData/badgeImageId/
escapeHtml are now exported from gpx.js so it doesn't have to
reimplement them — the same three helpers a second overlay would also
need.

views/index.erb

@@ -58,19 +58,29 @@
   <input type="file" class="gpx-file-input" accept=".gpx,.fit,.geojson,.zip" multiple hidden>
   <p class="gpx-upload__status" hidden></p>
 
-  <!-- Not a real file in the tree below: a live layer, refetched from the
-       offline Wikipedia ZIM (via kiwix-serve) for whatever's on screen as
-       the map pans/zooms, not a saved .geojson. Hidden entirely when no
-       kiwix-serve is configured (see gpx.js's mount()). -->
-  <label class="gpx-panel__wiki" hidden>
-    <input type="checkbox" class="gpx-panel__wiki-toggle">
-    <span>Wikipedia</span>
-  </label>
-
   <div class="gpx-tree"></div>
   <div class="gpx-legend" hidden></div>
 </div>
 
+<div id="overlays-panel" class="basemap-panel" hidden>
+  <div class="basemap-panel__header">
+    <strong>Overlays</strong>
+    <button type="button" class="basemap-panel__close" aria-label="Close">&times;</button>
+  </div>
+  <!-- Each item is a live layer refetched from its own source as the map
+       moves, not a file to add/remove/reorder — a plain on/off list, no
+       upload/new-folder actions the way Tracks & Points has. Wikipedia is
+       the first; see public/js/overlays.js for how one of these works. -->
+  <ul class="overlays-panel__list">
+    <li class="overlays-panel__item">
+      <label>
+        <input type="checkbox" class="overlays-panel__wiki-toggle">
+        <span>Wikipedia</span>
+      </label>
+    </li>
+  </ul>
+</div>
+
 <div id="livetrack-panel" class="basemap-panel" hidden>
   <div class="basemap-panel__header">
     <strong>LiveTrack sessions</strong>