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/components/FromNow/FromNow.js

@@ -18,13 +18,9 @@ class FromNow extends Component {
     };
   }
 
-  UNSAFE_componentWillReceiveProps(nextProps) {
-    if (this.props.when !== nextProps.when) {
-      // console.log(`Next when ${nextProps.when}`);
-      this.setState({
-        when: nextProps.when
-      });
-    }
+  // state.when just mirrors props.when (was UNSAFE_componentWillReceiveProps)
+  static getDerivedStateFromProps(props, state) {
+    return props.when !== state.when ? { when: props.when } : null;
   }
 
   shouldComponentUpdate(nextProps, nextState) {
@@ -36,7 +32,6 @@ class FromNow extends Component {
   }
 
   render() {
-    console.log(`Render from now ${this.state.when}`);
     return (
       <span data-toggle="tooltip" className="from-now" title={this.thatIs()}>{this.state.when}</span>
     );
@@ -49,9 +44,6 @@ FromNow.propTypes = {
   whenDate: PropTypes.instanceOf(Date)
 };
 
-FromNow.defaultProps = {
-};
-
 /*
  * const FromNowContainer = withTracker((props) => {
  *