feat(weather): OWM-Fallback über Server-.env wenn kein User-Key

Wetter-Proxy auf /api/weather/current nutzt optionalen Nutzer-Key aus
den Einstellungen, sonst OpenWeatherMapAPIKey aus der Umgebung.

Co-authored-by: Cursor <cursoragent@cursor.com>

client/src/i18n/locales/en.json

@@ -300,8 +300,8 @@
       "save": "Save Configuration",
       "saving": "Saving...",
       "saved": "Settings saved successfully!",
-      "key_help": "An API key is required to automatically fetch real-time weather and sea state parameters based on your vessel's GPS coordinates.",
-      "no_key": "Please set your OpenWeatherMap API Key in settings to enable weather auto-fill.",
+      "key_help": "Optional: your own OpenWeatherMap API key. If left empty, the operator-configured server key is used.",
+      "no_key": "No OpenWeatherMap API key available. Add your own key in settings or contact the operator.",
       "weather_success": "Weather details fetched successfully!",
       "weather_error": "Failed to fetch weather. Check your API key and connection.",
       "weather_date_mismatch": "Weather data can only be fetched for today ({{today}}). This logbook entry is dated {{date}}.",