Cache the weather forecast for offline fallback

Radar flyout now also shows a 3-day Open-Meteo forecast (free, no API
key) for the map's current center. Radar frames and the forecast fetch
independently — one being unreachable doesn't hide the other. The last
successful forecast is cached in localStorage and shown (with a
"cached from" note) when the live fetch fails, so there's still
something useful once actually offline.

Radar tile images themselves aren't cached — that needs a Service
Worker, left as a separate follow-up (see TODO.adoc).

TODO.adoc

@@ -15,7 +15,8 @@ independence from connectivity.
 * [ ] 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
-* [ ] Cache weather and precipitation radar when temporarily connected (4G/5G/Wi-Fi)
+* [x] Cache weather forecast when temporarily connected — Open-Meteo 3-day forecast in the Radar flyout, last successful fetch cached in localStorage as a fallback when offline
+** [ ] Cache the radar tile *images* too, not just the forecast text — needs a Service Worker (own bigger task: registration, cache strategy, invalidation); app has HTTPS everywhere (reverse proxy, incl. the in-van local server) so it's viable, just not done yet
 
 == NPS visual style