Map: dark backdrop + reliable tile caching

Two fixes to the dark map + offline tiles:
- Dark backdrop: body.dark-map also darkens .leaflet-container (was #ddd), so
  the light Leaflet background no longer flashes bright between/behind tiles
  while zooming or where a tile is missing.
- Tile caching: createTile fetched the display <img> from the URL but cached
  via a SEPARATE fetch — on a flaky mobile link that second request often
  failed, so tiles showed but were never cached and re-zooming re-fetched them.
  Now one fetch caches and displays from the same blob, so every shown tile is
  cached. CSP already allows connect-src to the OSM tile host.

Verified on device: container bg rgb(27,31,36) under dark map; IndexedDB tile
cache populated.

frontend/css/styles.css

@@ -1149,6 +1149,9 @@ select {
 /* Dark map: invert ONLY the tile layer (overlays — track, route, roads, puck — keep
    their colours). User-tuned filter on OSM standard tiles. Applies to every Leaflet map. */
 body.dark-map .leaflet-tile-pane { filter: invert(1) hue-rotate(180deg) brightness(1.5); }
+/* Darken the map backdrop too, else the light Leaflet container (#ddd) flashes bright
+   between/behind tiles while zooming or where a tile is missing. */
+body.dark-map .leaflet-container { background: #1b1f24; }
 /* Always-on compass needle: red half = North, grey = South. Rotates to point to real north.
    Top-RIGHT (zoom controls are top-left), below the status-bar safe area. */
 /* Position/direction puck (SVG divIcon) — no default leaflet-div-icon box. */