Weather: default to 5-day forecast, add weekday+date header line

- forecast_days now defaults to 5 (was 3) across default_settings(),
  check()'s fallback, and the settings form's pre-selected option.
- New show_date setting (on by default): weekday + date in the same
  "Samstag, 25. Juli" format Clock already used. Combined with the
  city name into one header line above the icon/temp block when both
  are on ("Münster · Samstag, 25. Juli"), each independently
  toggleable.
- Moved the weekday/month name lists and the "Weekday, D. Month"
  formatting out of clock.py into render_utils.format_date_de(), so
  Weather reuses the exact same formatting instead of a second copy
  that could drift.

web/templates/settings_weather.html

@@ -24,8 +24,8 @@
     <div class="field">
       <label for="forecast_days">Days to show</label>
       <select id="forecast_days" name="forecast_days">
-        <option value="3" {{ "selected" if settings.forecast_days == 3 or not settings.forecast_days }}>3 days</option>
-        <option value="5" {{ "selected" if settings.forecast_days == 5 }}>5 days</option>
+        <option value="3" {{ "selected" if settings.forecast_days == 3 }}>3 days</option>
+        <option value="5" {{ "selected" if settings.forecast_days == 5 or not settings.forecast_days }}>5 days</option>
         <option value="7" {{ "selected" if settings.forecast_days == 7 }}>7 days</option>
       </select>
     </div>
@@ -39,6 +39,10 @@
         <input type="checkbox" name="show_city" {{ "checked" if settings.show_city != False }}>
         City name
       </label>
+      <label class="checkbox">
+        <input type="checkbox" name="show_date" {{ "checked" if settings.show_date != False }}>
+        Weekday + date
+      </label>
       <label class="checkbox">
         <input type="checkbox" name="show_hourly_graph" {{ "checked" if settings.show_hourly_graph != False }}>
         Hourly temperature graph