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.

app.rb

@@ -58,6 +58,7 @@ module Maptool
     DATA_DIR = ENV.fetch("MAPTOOL_DATA_DIR") { File.join(__dir__, "data") }
 
     OFFLINE_PMTILES_PATH = File.join(DATA_DIR, "offline.pmtiles")
+    TERRAIN_PMTILES_PATH = File.join(DATA_DIR, "terrain.pmtiles")
     GPX_STORE = GpxStore.new(File.join(DATA_DIR, "gpx"))
     LIVETRACK_STORE = LiveTrackStore.new(File.join(DATA_DIR, "livetrack.json"))
 
@@ -185,6 +186,19 @@ module Maptool
       send_file OFFLINE_PMTILES_PATH, type: "application/octet-stream"
     end
 
+    # Same optional-local-file story as offline.pmtiles above — see
+    # docs/development.adoc for how to generate one, until then hillshade
+    # just stays unavailable while the offline basemap is active.
+    get "/api/terrain-tiles" do
+      json available: File.exist?(TERRAIN_PMTILES_PATH)
+    end
+
+    get "/tiles/terrain.pmtiles" do
+      halt 404 unless File.exist?(TERRAIN_PMTILES_PATH)
+
+      send_file TERRAIN_PMTILES_PATH, type: "application/octet-stream"
+    end
+
     get "/api/gpx/tree" do
       json GPX_STORE.tree
     end