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.

views/layout.erb

@@ -31,6 +31,7 @@
     {
       "imports": {
         "/js/auth.js": "/js/auth.js?v=<%= ASSET_VERSION %>",
+        "/js/dem.js": "/js/dem.js?v=<%= ASSET_VERSION %>",
         "/js/gnss.js": "/js/gnss.js?v=<%= ASSET_VERSION %>",
         "/js/gpx.js": "/js/gpx.js?v=<%= ASSET_VERSION %>",
         "/js/icons.js": "/js/icons.js?v=<%= ASSET_VERSION %>",
@@ -42,6 +43,7 @@
         "/js/panels.js": "/js/panels.js?v=<%= ASSET_VERSION %>",
         "/js/plugins.js": "/js/plugins.js?v=<%= ASSET_VERSION %>",
         "/js/radar.js": "/js/radar.js?v=<%= ASSET_VERSION %>",
+        "/js/shadow.js": "/js/shadow.js?v=<%= ASSET_VERSION %>",
         "/js/sun.js": "/js/sun.js?v=<%= ASSET_VERSION %>",
         "/js/track_profile.js": "/js/track_profile.js?v=<%= ASSET_VERSION %>"
       }