feat: add OWM alerts, UV index support, and provider info UI

- Parse OWM One Call 3.0 weather alerts and map to Warning structs
- Map hourly UVI from OWM response to HourlyForecast.UVIndex
- Add severity helper mapping OWM alert tags to severity levels
- Extract UVIndex through compute layer to timeline slots
- Smart warnings: use provider-supplied alerts, fall back to DWD
- Show UV index in dashboard timeline tooltips
- Add provider description below forecast dropdown with i18n

web/templates/setup.html

@@ -391,6 +391,7 @@
             <input type="password" id="forecast-api-key" placeholder="{{t "setup.forecast.apiKeyPlaceholder"}}" class="w-full px-3 py-1.5 rounded-lg border border-gray-300 dark:border-gray-600 bg-white dark:bg-gray-700 text-sm">
           </div>
         </div>
+        <p id="forecast-provider-desc" class="text-xs text-gray-500 dark:text-gray-400">{{t "setup.forecast.providerDesc.openmeteo"}}</p>
         <div id="forecast-provider-hint" class="text-xs text-gray-400"></div>
         <button type="submit" class="px-4 py-1.5 text-sm bg-orange-600 text-white rounded-lg hover:bg-orange-700 transition">{{t "setup.forecast.saveConfig"}}</button>
       </form>