Switch to vector maps (MapLibre GL JS + PMTiles) and add RainViewer radar

Replace Leaflet/raster tiles with MapLibre GL JS: an online OpenFreeMap
style and an offline style built from a local PMTiles archive via
pmtiles.js and protomaps-themes-base (no glyphs/labels offline by
design, see docs/architecture.adoc). Frontend libraries are vendored
locally rather than loaded from a CDN so the app keeps working with no
internet access.

Also add an animated RainViewer precipitation radar overlay (toggle,
play/pause, scrub slider), matching the default time window of the
weather-radar-card Home Assistant integration.

app.rb

@@ -6,6 +6,8 @@ module Maptool
     set :public_folder, File.join(__dir__, "public")
     set :views, File.join(__dir__, "views")
 
+    OFFLINE_PMTILES_PATH = File.join(__dir__, "data", "offline.pmtiles")
+
     get "/" do
       erb :index
     end
@@ -13,5 +15,18 @@ module Maptool
     get "/api/health" do
       json status: "ok"
     end
+
+    get "/api/offline-tiles" do
+      json available: File.exist?(OFFLINE_PMTILES_PATH)
+    end
+
+    # Served as a plain range-request-capable file, not decoded server-side:
+    # pmtiles.js reads the archive's directory and fetches only the byte
+    # ranges it needs for the tiles currently on screen.
+    get "/tiles/offline.pmtiles" do
+      halt 404 unless File.exist?(OFFLINE_PMTILES_PATH)
+
+      send_file OFFLINE_PMTILES_PATH, type: "application/octet-stream"
+    end
   end
 end