Add an offline terrain shadow overlay to the Overlays panel

Shades the visible map by whether each point is currently blocked from
the sun by surrounding terrain (shademap.app-style), computed fully
offline via the same Terrarium DEM tiles the Terrain shading layer
already renders, plus this session's sun-position math and Radar's
checkbox+expandable UI shape.

dem.js pulls elevation-at-a-point out on its own since TODO.adoc's
slope readout and solar calculator will need the same lookup.

public/css/style.css

@@ -242,6 +242,27 @@ html, body {
   font-size: 11px;
 }
 
+.shadow-control__time {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+}
+
+.shadow-control__slider {
+  width: 110px;
+}
+
+.shadow-control__clock {
+  font-variant-numeric: tabular-nums;
+  white-space: nowrap;
+}
+
+.shadow-control__hint {
+  margin: 6px 0 0;
+  color: #888;
+  font-size: 11px;
+}
+
 .basemap-panel {
   position: fixed;
   top: 0;