Use station data for current conditions; persist keys server-side

Station overlay:
- When Ambient Weather station data is present and fresh (<15 min),
  its readings replace the model's current conditions: temp, feels-like,
  humidity, dewpoint, wind speed/direction, gusts, pressure (inHg→hPa),
  UV index, rain today, wind peek summary, and data-panel readouts
- A small "📡 My Station" badge appears in the hero when active
- applyStationOverlay() is called at the end of renderWeather (instant
  if station data already loaded) and again whenever station data
  refreshes (so the display stays live every 60 s)
- Hourly/daily forecasts continue to use Open-Meteo

Server-side key persistence:
- POST /api/ambient-config saves keys to data/ambient-config.json
  (same persistent volume as VAPID and subscriptions)
- GET /api/ambient-config returns {configured: bool} without exposing keys
- /api/ambient checks query params → saved config → env vars in order
- Settings Save button posts keys to server after a successful test;
  confirms "saved for all devices" in the status line
- On page load, if no localStorage keys but server is configured,
  the station loads automatically on any device

app/public/style.css

@@ -2487,3 +2487,18 @@ body.map-fs #unified-map {
 }
 .station-name { font-weight: 600; font-size: 0.95rem; }
 .station-updated { font-size: 0.75rem; color: var(--ink-light); }
+
+.station-badge {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.3rem;
+  margin-top: 0.4rem;
+  font-size: 0.72rem;
+  font-weight: 600;
+  color: var(--accent);
+  background: color-mix(in srgb, var(--accent) 12%, transparent);
+  border: 1px solid color-mix(in srgb, var(--accent) 30%, transparent);
+  border-radius: 999px;
+  padding: 0.15rem 0.55rem;
+  letter-spacing: 0.02em;
+}