[weather] feat: add configurable forecast date format option (#3918)

I was a little disappointed that I couldn't change the date format in
the forecast because it was hard-coded. This PR introduces a new option
(`forecastDateFormat`) that allows the user to specify the format
themselves. The default remains `ddd`.

## Before

<img width="483" height="524" alt="Screenshot From 2025-10-18 18-26-13"
src="https://github.com/user-attachments/assets/2de6af55-e73c-42e8-a3fe-7386ef5f90e0"
/>

## After (examples)

### `forecastDateFormat: "dddd"`
<img width="483" height="524" alt="Screenshot From 2025-10-18 18-26-27"
src="https://github.com/user-attachments/assets/cd86c798-f1e4-4d75-adf9-c4e549aa2a51"
/>

### `forecastDateFormat: "ddd, D MMM"`
<img width="483" height="524" alt="Screenshot From 2025-10-18 18-28-02"
src="https://github.com/user-attachments/assets/79aaa7b3-810a-4ab1-833c-09dfab7f457a"
/>

modules/default/weather/forecast.njk

@@ -16,7 +16,7 @@
         {% elif (currentStep == 1) and config.ignoreToday == false and config.absoluteDates == false %}
           <td class="day">{{ "TOMORROW" | translate }}</td>
         {% else %}
-          <td class="day">{{ f.date.format("ddd") }}</td>
+          <td class="day">{{ f.date.format(config.forecastDateFormat) }}</td>
         {% endif %}
         <td class="bright weather-icon">
           <span class="wi weathericon wi-{{ f.weatherType }}"></span>