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/icons.js

@@ -7,6 +7,16 @@ export const LAYERS_ICON = `
   <path d="M2 14l10 6 10-6"/>
 </svg>`;
 
+// Two overlapping circles — distinct from LAYERS_ICON's stacked-planes
+// look (that one picks a *base* map; this one is for the sidebar flyout
+// toggling live overlay layers, e.g. Wikipedia, on top of whichever base
+// map is showing) while still reading as "layers combining."
+export const OVERLAYS_ICON = `
+<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linejoin="round" stroke-linecap="round">
+  <circle cx="9" cy="12" r="6"/>
+  <circle cx="15" cy="12" r="6"/>
+</svg>`;
+
 export const HILLSHADE_ICON = `
 <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linejoin="round" stroke-linecap="round">
   <path d="M2 18 9 7l3.5 5.5L15 9l7 9z"/>