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/api/client.js

@@ -166,12 +166,8 @@ export const reservationsApi = {
   delete: (tripId, id) => apiClient.delete(`/trips/${tripId}/reservations/${id}`).then(r => r.data),
 }
 
-export const exchangeApi = {
-  getRates: () => apiClient.get('/exchange-rates').then(r => r.data),
-}
-
 export const weatherApi = {
-  get: (lat, lng, date) => apiClient.get('/weather', { params: { lat, lng, date, units: 'metric' } }).then(r => r.data),
+  get: (lat, lng, date) => apiClient.get('/weather', { params: { lat, lng, date } }).then(r => r.data),
 }
 
 export const settingsApi = {