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.

public/css/style.css

@@ -1214,6 +1214,47 @@ html, body {
   background: #f0f0f0;
 }
 
+.overlays-panel__icon {
+  display: flex;
+  width: 16px;
+  height: 16px;
+  flex-shrink: 0;
+  color: #555;
+}
+
+.overlays-panel__icon svg {
+  width: 100%;
+  height: 100%;
+}
+
+.overlays-panel__row {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 6px 4px;
+}
+
+.overlays-panel__name {
+  border: none;
+  background: none;
+  padding: 0;
+  font: inherit;
+  color: inherit;
+  text-align: left;
+  cursor: pointer;
+}
+
+.overlays-panel__name:hover {
+  text-decoration: underline;
+}
+
+.overlays-panel__details {
+  margin: 0 4px 6px 26px;
+  padding-top: 6px;
+  border-top: 1px solid #eee;
+  font-size: 12px;
+}
+
 .livetrack-panel__list {
   flex: 1;
   overflow-y: auto;