Show current weather on the Overview

Add a weather card to the Web App Overview, wired like the OpenSky live-map
overlay and backed by the OpenWeather plugin.

- API Server: GET /api/integrations/openweather/current resolves the
  caller's cascade, gates it (master/org/personal opt-in/key), and returns
  trimmed current conditions for the configured or a supplied ?lat=&lon=
  point; off/keyless returns {unavailable, detail} so the card degrades.
- BFF relay (forwards the location override) + route; api.js client.
- Dashboard: a Weather card stacked above Schedule showing an emoji
  condition, temperature in the resolved unit, description, location, and a
  feels-like/wind/humidity/cloud grid. Location follows the same cascade as
  the map (drone -> phone -> browser -> default) without prompting for geo;
  refreshes every 10 min while on Overview.
- validLatLon test; rebuilt embedded frontend.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

API Server/internal/api/integrations_openweather_test.go

@@ -98,6 +98,21 @@ func TestOpenWeatherViewMasksKey(t *testing.T) {
 	}
 }
 
+func TestValidLatLon(t *testing.T) {
+	ok := [][2]string{{"0", "0"}, {"52.2297", "21.0122"}, {"-90", "180"}, {"90", "-180"}}
+	for _, c := range ok {
+		if !validLatLon(c[0], c[1]) {
+			t.Errorf("validLatLon(%q,%q) = false, want true", c[0], c[1])
+		}
+	}
+	bad := [][2]string{{"", ""}, {"91", "0"}, {"0", "181"}, {"-91", "0"}, {"abc", "0"}, {"0", "x"}}
+	for _, c := range bad {
+		if validLatLon(c[0], c[1]) {
+			t.Errorf("validLatLon(%q,%q) = true, want false", c[0], c[1])
+		}
+	}
+}
+
 // mergeOpenWeather must preserve sibling plugin keys (opensky/webdav) untouched.
 func TestMergeOpenWeatherPreservesSiblings(t *testing.T) {
 	existing := json.RawMessage(`{"opensky":{"enabled":true},"webdav":{"config":{"baseURL":"https://x"}}}`)