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).

public/css/style.css

@@ -193,6 +193,36 @@ html, body {
   white-space: nowrap;
 }
 
+.radar-control__forecast {
+  margin-top: 6px;
+  padding-top: 6px;
+  border-top: 1px solid #eee;
+}
+
+.radar-control__forecast-day {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 2px 0;
+  white-space: nowrap;
+}
+
+.radar-control__forecast-day span:first-child {
+  width: 2.2em;
+  color: #888;
+}
+
+.radar-control__forecast-day span:nth-child(3) {
+  margin-left: auto;
+  font-variant-numeric: tabular-nums;
+}
+
+.radar-control__forecast-stale {
+  margin: 4px 0 0;
+  color: #888;
+  font-size: 11px;
+}
+
 .basemap-panel {
   position: fixed;
   top: 0;