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.

TODO.adoc

@@ -9,8 +9,9 @@ independence from connectivity.
 == Offline data and cartography
 
 * [ ] Offline vector tiles (PMTiles/MBTiles) for Europe/world via Planetiler or MapTiler
-* [ ] Shaded relief / DEM layer (terrain slope for parking)
-* [ ] Hillshade & contour lines from OSM for hikes and mountain passes
+* [x] Shaded relief / DEM layer — hillshade toggle in the sidebar, sourced from public Terrarium tiles online; reads `data/terrain.pmtiles` for offline if one's been generated (see docs/development.adoc), otherwise the toggle is online-only for now
+** [ ] Terrain slope readout for parking (numeric, not just visual shading)
+* [ ] Contour lines from OSM for hikes and mountain passes
 * [ ] Overpass/OSM scripts to extract POIs (drinking water, dump stations, LPG, public toilets, laundromats, shops, springs)
 * [ ] Import Park4Night / iOverlander / Freecampsites data
 * [ ] Kiwix/ZIM offline Wikipedia linked to POI GPS coordinates
@@ -25,7 +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 combined with offline hillshade for a characteristic relief look
+** [ ] Elevation-tinted terrain (color by height band, not just shading — see the hillshade toggle above)
 ** [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)