cleanup: defaultProps -> JS default params on our function components

React 19 drops defaultProps on function components. Converted the 9 of
ours that used it (App, Navigation, ReSendEmail, Reconnect, OAuthLoginButton,
PageHeader, Page, EditDocument, EditSubscription) to destructured default
params; App defaults userId/emailAddress in its withTracker instead (it
spreads {...props} widely). Class components keep defaultProps (still
supported). The only defaultProps warnings left are from the react-share
npm package (CreatedButton/Icon), not our code. REST smoke byte-identical.

imports/ui/components/PageHeader/PageHeader.js

@@ -3,7 +3,7 @@ import PropTypes from 'prop-types';
 
 import './PageHeader.scss';
 
-const PageHeader = ({ title, subtitle }) => (
+const PageHeader = ({ title, subtitle = '' }) => (
   <div className="PageHeader">
     <div className="PageHeader-container">
       <h1>{title}</h1>
@@ -12,10 +12,6 @@ const PageHeader = ({ title, subtitle }) => (
   </div>
 );
 
-PageHeader.defaultProps = {
-  subtitle: '',
-};
-
 PageHeader.propTypes = {
   title: PropTypes.string.isRequired,
   subtitle: PropTypes.string,