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.

docs/development.adoc

@@ -43,6 +43,22 @@ build one locally:
 Once the file is in place, `GET /api/offline-tiles` reports
 `available: true` and the map defaults to the offline style.
 
+== Offline terrain data (hillshade)
+
+The sidebar's "Terrain shading" toggle works online out of the box
+(public https://github.com/tilezen/joerd[Terrarium] elevation tiles,
+no API key). For it to work offline too, place a PMTiles archive of
+Terrarium-encoded `raster-dem` tiles at `data/terrain.pmtiles`
+(`GET /api/terrain-tiles` reports `available: true` once it's there,
+same pattern as offline map data above). It is not part of the
+repository — build one locally from a DEM source (e.g. Copernicus
+GLO-30 or SRTM) with a tool like
+https://github.com/mapbox/rio-rgbify[rio-rgbify] (GeoTIFF ->
+Terrarium-encoded PNG tiles in an `.mbtiles`), then
+`pmtiles convert out.mbtiles data/terrain.pmtiles` with
+https://github.com/protomaps/go-pmtiles[go-pmtiles], same as
+`data/offline.pmtiles`.
+
 == GNSS position (gpsd)
 
 The "GNSS" control needs https://gpsd.io[gpsd] running and pointed at