cleanup: UNSAFE_componentWillReceiveProps -> modern lifecycles (React 19)

The 4 class components on UNSAFE_componentWillReceiveProps converted:
- FromNow, FireStats: state is a pure mirror of props -> getDerivedStateFromProps
- Fires: split the derived state (getDerivedStateFromProps) from the URL-
  canonicalization side effect (componentDidUpdate, guarded by prevProps) so
  the side effect never runs inside the pure gDSFP
- SelectionMap: marker is also locally draggable, so gDSFP would clobber a
  drag -> componentDidUpdate guarded on a real center/distance value change
  (no clobber, no setState loop), merged with the existing fit()

Browser-verified: /fires count, fire detail + FromNow, active-fire URL
canonicalizes to /fire/archive/:id, home SelectionMap renders stably (no
render loop). No UNSAFE_ warnings remain from our code. REST smoke
byte-identical.

imports/ui/pages/FiresMap/FireStats.js

@@ -17,17 +17,19 @@ class FireStats extends Component {
     };
   }
 
-  UNSAFE_componentWillReceiveProps(nextProps) {
-    if (this.props.lastCheck !== nextProps.lastCheck ||
-        this.props.lastFireDetected !== nextProps.lastFireDetected ||
-        this.props.loadingAll !== nextProps.loadingAll
+  // state mirrors props (was UNSAFE_componentWillReceiveProps)
+  static getDerivedStateFromProps(props, state) {
+    if (props.lastCheck !== state.lastCheck ||
+        props.lastFireDetected !== state.lastFireDetected ||
+        props.loadingAll !== state.loadingAll
     ) {
-      this.setState({
-        lastFireDetected: nextProps.lastFireDetected,
-        lastCheck: nextProps.lastCheck,
-        loadingAll: nextProps.loadingAll
-      });
+      return {
+        lastFireDetected: props.lastFireDetected,
+        lastCheck: props.lastCheck,
+        loadingAll: props.loadingAll
+      };
     }
+    return null;
   }
 
   shouldComponentUpdate(nextProps, nextState) {
@@ -54,7 +56,4 @@ FireStats.propTypes = {
   lastCheck: PropTypes.instanceOf(Date)
 };
 
-FireStats.defaultProps = {
-};
-
 export default withTranslation()(FireStats);