deps: react-router-dom 4 -> 6 (history 5, react-router-bootstrap 0.26)

Removes the Router/Switch/Route/Link/LinkContainer legacy-context
warnings. Keeps the shared history singleton (used outside React by
NotificationsObserver + Utils/location) via unstable_HistoryRouter, and
bridges v6 hooks back to v4-shaped history/match/location props with a
new withRouterCompat HOC so the ~20 class pages stay untouched.

- <Switch> -> <Routes>, component= -> element=
- Authenticated/Public -> guard components (children or <Navigate replace>)
- LocationListener -> useLocation + useEffect function
- regex route /fire/:type(active|archive|alert)/:id -> /fire/:type/:id
- history.listen callback is ({ location }) in history v5

Browser-verified: SPA nav, /subscriptions->/login auth redirect,
deep-link /fire/archive/:id (params), browser back/forward. REST smoke
byte-identical.

imports/ui/components/Authenticated/Authenticated.js

@@ -1,25 +1,16 @@
 import React from 'react';
 import PropTypes from 'prop-types';
-import { Route, Redirect } from 'react-router-dom';
+import { Navigate } from 'react-router-dom';
 
-const Authenticated = ({ loggingIn, authenticated, component, path, exact, ...rest }) => (
-  <Route
-    path={path}
-    exact={exact}
-    render={props => (
-      authenticated ?
-        (React.createElement(component, { ...props, ...rest, loggingIn, authenticated })) :
-        (<Redirect to="/login" />)
-    )}
-  />
+// v6 route guard: rendered as a route `element` wrapping the real page.
+// Renders its children when authenticated, otherwise redirects to /login.
+const Authenticated = ({ authenticated, children }) => (
+  authenticated ? children : <Navigate to="/login" replace />
 );
 
 Authenticated.propTypes = {
-  loggingIn: PropTypes.bool.isRequired,
   authenticated: PropTypes.bool.isRequired,
-  // elementType (not func): route components may be HOC-wrapped (translate(),
-  // memo, forwardRef), which are objects, not plain functions.
-  component: PropTypes.elementType.isRequired,
+  children: PropTypes.node.isRequired
 };
 
 export default Authenticated;