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/withRouterCompat/withRouterCompat.js

@@ -0,0 +1,33 @@
+import React from 'react';
+import { useNavigate, useLocation, useParams } from 'react-router-dom';
+
+// react-router v6 dropped the `history`/`match`/`location` props that our ~20
+// class page components still read. Rather than convert every class to hooks,
+// this HOC bridges the v6 hooks back to the v4-shaped props those components
+// expect: `history.push/replace(to, state)`, `match.params`, `match.url`,
+// and `location`. New code should use the hooks directly instead.
+export default function withRouterCompat(Component) {
+  function WithRouterCompat(props) {
+    const navigate = useNavigate();
+    const location = useLocation();
+    const params = useParams();
+
+    const history = {
+      push: (to, state) => navigate(to, state !== undefined ? { state } : undefined),
+      replace: (to, state) => navigate(to, { replace: true, ...(state !== undefined ? { state } : {}) }),
+      goBack: () => navigate(-1),
+      goForward: () => navigate(1),
+      location
+    };
+
+    // v4 `match.url` was the matched path; for our leaf/exact routes that is
+    // simply the current pathname.
+    const match = { params, url: location.pathname, path: location.pathname };
+
+    return <Component {...props} history={history} location={location} match={match} />;
+  }
+
+  const name = Component.displayName || Component.name || 'Component';
+  WithRouterCompat.displayName = `withRouterCompat(${name})`;
+  return WithRouterCompat;
+}