Add a hillshade/terrain-shading toggle to the map

Sidebar toggle (new HillshadeControl) adds a raster-dem hillshade
layer: public Terrarium tiles when online, or a local
data/terrain.pmtiles when present (mirrors offline.pmtiles' optional-
local-file pattern, new /api/terrain-tiles + /tiles/terrain.pmtiles
endpoints). No offline terrain data is bundled, so the toggle is
online-only until one is generated — see docs/development.adoc for
how.

public/js/icons.js

@@ -7,6 +7,12 @@ export const LAYERS_ICON = `
   <path d="M2 14l10 6 10-6"/>
 </svg>`;
 
+export const HILLSHADE_ICON = `
+<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linejoin="round" stroke-linecap="round">
+  <path d="M2 18 9 7l3.5 5.5L15 9l7 9z"/>
+  <path d="M2 18h20"/>
+</svg>`;
+
 export const GPX_ICON = `
 <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linecap="round" stroke-linejoin="round">
   <circle cx="5" cy="6" r="2"/>