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/css/style.css

@@ -373,21 +373,6 @@ 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
@@ -1189,6 +1174,27 @@ html, body {
   box-shadow: 0 0 0 2px rgba(42, 157, 143, 0.4);
 }
 
+.overlays-panel__list {
+  flex: 1;
+  overflow-y: auto;
+  list-style: none;
+  margin: 0;
+  padding: 8px 12px;
+}
+
+.overlays-panel__item label {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 6px 4px;
+  border-radius: 4px;
+  cursor: pointer;
+}
+
+.overlays-panel__item label:hover {
+  background: #f0f0f0;
+}
+
 .livetrack-panel__list {
   flex: 1;
   overflow-y: auto;