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/js/map.js

@@ -547,7 +547,10 @@ fetch("/api/offline-tiles")
 
 fetch("/api/terrain-tiles")
   .then((response) => response.json())
-  .then(({ available }) => overlaysControl.setHillshadeOfflineAvailable(available))
+  .then(({ available }) => {
+    overlaysControl.setHillshadeOfflineAvailable(available);
+    overlaysControl.setShadowOfflineAvailable(available);
+  })
   .catch(() => {});
 
 fetch("/api/kiwix")