v2.5.6: Open-Meteo weather, WebSocket fixes, admin improvements

- Replace OpenWeatherMap with Open-Meteo (no API key needed)
  - 16-day forecast (up from 5 days)
  - Historical climate averages as fallback beyond 16 days
  - Auto-upgrade from climate to real forecast when available
- Fix Vacay WebSocket sync across devices (socket-ID exclusion instead of user-ID)
- Add GitHub release history tab in admin panel
- Show cluster count "1" for single map markers when zoomed out
- Add weather info panel in admin settings (replaces OpenWeatherMap key input)
- Update i18n translations (DE + EN)

client/src/components/Map/MapView.jsx

@@ -232,6 +232,7 @@ export function MapView({
         spiderfyOnMaxZoom
         showCoverageOnHover={false}
         zoomToBoundsOnClick
+        singleMarkerMode
         iconCreateFunction={(cluster) => {
           const count = cluster.getChildCount()
           const size = count < 10 ? 36 : count < 50 ? 42 : 48