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/icons.js

@@ -159,6 +159,15 @@ export const RADAR_CLOUD_ICON = `
   <rect x="14.4" y="20" width="1.6" height="2"/>
 </svg>`;
 
+// Hand-drawn, not NPS-sourced — same story as RADAR_CLOUD_ICON above, no
+// terrain-shadow/sun-behind-a-ridge pictogram in that set either. Used as
+// the Overlays panel's Terrain shadow glyph (shadow.js via overlays.js).
+export const TERRAIN_SHADOW_ICON = `
+<svg viewBox="0 0 22 22" fill="currentColor">
+  <circle cx="16" cy="6" r="3"/>
+  <polygon points="0,19 6,9 9,13 13,6 22,19"/>
+</svg>`;
+
 // src/bicycle-trail-22.svg — pulled out to its own constant since it's also
 // used unrasterized, inline, as PARK4NIGHT_TAGS.vtt below.
 const BICYCLE_TRAIL_ICON = `