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.

public/js/map.js

@@ -5,7 +5,8 @@ import {
   addProtocol,
 } from "/vendor/maplibre-gl/maplibre-gl.mjs";
 import { RadarControl } from "/js/radar.js";
-import { GpxPanelControl, setKiwixConfig } from "/js/gpx.js";
+import { GpxPanelControl } from "/js/gpx.js";
+import { OverlaysControl } from "/js/overlays.js";
 import { GnssControl } from "/js/gnss.js";
 import { LiveTrackPanelControl } from "/js/livetrack.js";
 import { TrackProfilePanel } from "/js/track_profile.js";
@@ -556,6 +557,8 @@ profilePanel.mount();
 
 sidebar.insertAdjacentHTML("beforeend", `<div class="sidebar-group-label" title="Explorer"></div>`);
 new GpxPanelControl(map, profilePanel).mount(sidebar);
+const overlaysControl = new OverlaysControl(map);
+overlaysControl.mount(sidebar);
 const hillshadeControl = new HillshadeControl(map);
 hillshadeControl.mount(sidebar);
 // Running a plugin needs a login (POST /api/plugins/:name/run) — same
@@ -615,7 +618,7 @@ fetch("/api/terrain-tiles")
 
 fetch("/api/kiwix")
   .then((response) => response.json())
-  .then(setKiwixConfig)
+  .then(({ available }) => overlaysControl.setWikiAvailable(available))
   .catch(() => {});
 
 // Alt+click anywhere shows every rendered feature at that point — base map