Map tiles: switch to Esri World Light Gray (OSM 403s file://)

tiles="OpenStreetMap" returned HTTP 403 "referer is required by tile usage
policy" in the target browser — OSM graylists refererless requests and a
karte.html opened via file:// sends no Referer. Esri's Canvas/World_Light_Gray
base + reference layers need neither a key nor a Referer and keep the muted
look. New MAP_TILES / MAP_TILES_LABELS / MAP_TILES_ATTR constants; generate_map
adds them as explicit TileLayers.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/improvements.md

@@ -163,11 +163,11 @@ or at least skip exact duplicates by default.
 ## 9. Offline map 🟢
 
 `karte.html` loads Leaflet/jQuery/Bootstrap from CDNs and tiles from
-`tile.openstreetmap.org` (`MAP_TILES`; switched away from CartoDB positron, which
-now needs an API key). The map is blank without internet even though all the
-data is local. If offline use matters, vendor the Leaflet JS/CSS into `data/`
-and post-process the Folium output to point at local files (tiles would still
-need caching or an offline tile pack — larger effort).
+`server.arcgisonline.com` (Esri; `MAP_TILES` — Carto needs a key, OSM 403s
+`file://`). The map is blank without internet even though all the data is local.
+If offline use matters, vendor the Leaflet JS/CSS into `data/` and post-process
+the Folium output to point at local files (tiles would still need caching or an
+offline tile pack — larger effort).
 
 ## 10. Small code-quality items 🟢 — partly done