Make weather update interval configurable, was hardcoded to 30 min

Screen.poll_interval was a fixed class attribute the scheduler read
directly -- added Screen.get_poll_interval(settings) so a screen can make
it settings-driven instead. WeatherScreen overrides it to read a new
"update_minutes" field (default 30, matching the old hardcoded value).
New number input on the Weather settings page.

Caught a real edge case while testing: `raw or 30` treats an explicit 0
as falsy and silently replaces it with 30 instead of clamping to the
1-minute floor. Not reachable via the UI (min="1" on the input) but the
method should still be correct standalone -- fixed with an explicit
None-check instead.

web/templates/settings_weather.html

@@ -29,6 +29,11 @@
         <option value="7" {{ "selected" if settings.forecast_days == 7 }}>7 days</option>
       </select>
     </div>
+    <div class="field">
+      <label for="update_minutes">Update every (minutes)</label>
+      <input type="number" id="update_minutes" name="update_minutes" min="1" value="{{ settings.update_minutes or 30 }}">
+      <p class="hint">Weather itself only refreshes the panel when something actually changed, so a shorter interval here mostly just means fresher data, not more flashing.</p>
+    </div>
   </div>
 
   <div class="card">