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/js/setup.js

@@ -995,13 +995,19 @@
   function toggleForecastApiKeyField(provider) {
     const group = document.getElementById("forecast-apikey-group");
     const hint = document.getElementById("forecast-provider-hint");
+    const desc = document.getElementById("forecast-provider-desc");
+    const ts = window.HG && window.HG.t ? window.HG.t : {};
+    const providerDesc = (ts.setup && ts.setup.forecast && ts.setup.forecast.providerDesc) || {};
     if (provider === "openweathermap") {
       group.classList.remove("hidden");
       hint.innerHTML = '<a href="https://openweathermap.org/api/one-call-3" target="_blank" rel="noopener" class="text-orange-600 hover:underline">Requires One Call 3.0 subscription \u2192</a>';
+      if (desc) desc.textContent = providerDesc.openweathermap || "48h hourly + 8-day daily forecast. Includes UV index and worldwide weather alerts.";
     } else {
       group.classList.add("hidden");
       hint.textContent = "";
+      if (desc) desc.textContent = providerDesc.openmeteo || "Free, no API key needed. 3-day hourly forecast (temp, humidity, wind, solar, pressure). Warnings via DWD (Germany only).";
     }
+    if (desc) desc.classList.remove("hidden");
   }
 
   document.getElementById("forecast-provider-select").addEventListener("change", (e) => {