Move Radar and Terrain shading into the Overlays panel

Both used to be their own separate sidebar icons; neither has
anything to manage beyond on/off, same as Wikipedia already got its
own Overlays entry for. Terrain shading is a plain checkbox item now.
Radar keeps its scrub/forecast controls, but behind an explicit
expand (clicking its name), separate from the checkbox — checking it
on shouldn't force the details open, and expanding to scrub/check the
forecast shouldn't also flip the layer on.

Both controls now build their own <li> (RadarControl.mount(li),
HillshadeControl.mount(li) — the latter moved into overlays.js
entirely, it has no reason to live in map.js once it's not building
its own sidebar icon) rather than a whole sidebar-item + flyout the
way they used to.

Also gave each Overlays item a leading icon: Wikipedia and Terrain
shading reuse existing NPS pictograms (WIKI_ICON, and WILDERNESS_ICON
newly exported from icons.js's forest entry — NPS has no dedicated
mountain/terrain glyph, wilderness-22.svg is the closest honest
match). Radar's is hand-drawn (a rain cloud) since NPS's set has
nothing weather-related to reuse.

views/index.erb

@@ -70,18 +70,15 @@
     <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>
+  <!-- Each item is a live layer refetched/toggled 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.
+       Populated by OverlaysControl#mount (public/js/overlays.js), not
+       here — Radar's item needs its own scrub/forecast markup built by
+       RadarControl itself (radar.js), so all three items go through the
+       same "control owns its own <li>" path rather than half static
+       markup, half dynamic. -->
+  <ul class="overlays-panel__list"></ul>
 </div>
 
 <div id="livetrack-panel" class="basemap-panel" hidden>