Add elevation-tinted terrain (hypsometric color-relief layer)

Reuses the same raster-dem source the hillshade toggle already added
— a color-relief layer tints by elevation band (green valleys through
brown mid-slopes to pale grey peaks, in this app's own palette) under
the hillshade layer, so one toggle gives the combined shaded-relief
look instead of two separate layers.

TODO.adoc

@@ -26,7 +26,7 @@ independence from connectivity.
 ** [x] Filter panel as a "Unigrid Matrix" — an icon grid to toggle categories on/off, styled like the legend on an official NPS paper map, instead of plain dropdowns
 * [ ] Custom vector style (MVT/MapLibre) using the NPS color palette — muted natural tones for forests, water, and contours — so POIs and routes stand out at high zoom
 ** [ ] Landuse & protections: distinct styling for national parks, nature reserves (translucent green/gray-green), military zones, and private land (important for wild camping)
-** [ ] Elevation-tinted terrain (color by height band, not just shading — see the hillshade toggle above)
+** [x] Elevation-tinted terrain — a `color-relief` layer (hypsometric tint, this app's own green→brown→gray palette) under the hillshade layer, same "Terrain shading" toggle
 ** [x] Road hierarchy: clearly distinguish paved roads from unpaved/gravel/track — critical for knowing if the camper can get there
 
 == Main menu reorganization (4 modules)