Let the shell say where home is, and drop the router from the call

The call reached for react-router in five places to send the user
"home": on leaving without a lobby, from the lobby's recents link, from
the post-call screen, from the error page's return button and from the
header logo. Home is the standalone app's home page; the call has no
idea where that is, and a component has no such place at all — its host
decides what follows a call. Yet the component had to mount a
MemoryRouter just so those hooks would not throw.

`useLeaveToHome` is the way home as the shell supplies it: the app
provides `navigate("/")` from inside its router, the component provides
nothing, and everything that used to link to "/" now either calls it or,
when there is none, offers no way out. The logo becomes a plain logo,
the recents and "not now" links disappear, the error page's button does
too. `ClientProvider`'s logout goes the same way. The component no
longer renders a router.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/button/LeaveToHomeLink.tsx

@@ -0,0 +1,40 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { type FC, type MouseEvent, type ReactNode, useCallback } from "react";
+import { Link } from "@vector-im/compound-web";
+
+import { useLeaveToHome } from "../LeaveToHomeContext";
+
+interface Props {
+  className?: string;
+  children: ReactNode;
+}
+
+/**
+ * A link out of the call, to wherever the user came from. Renders nothing when
+ * there is nowhere to go (see {@link useLeaveToHome}).
+ */
+export const LeaveToHomeLink: FC<Props> = ({ className, children }) => {
+  const leaveToHome = useLeaveToHome();
+  const onClick = useCallback(
+    (e: MouseEvent<HTMLAnchorElement>) => {
+      e.preventDefault();
+      leaveToHome?.();
+    },
+    [leaveToHome],
+  );
+
+  if (leaveToHome === null) return null;
+  // Where this leads is the shell's business, so the link has no address of
+  // its own to offer
+  return (
+    <Link className={className} href="#" onClick={onClick}>
+      {children}
+    </Link>
+  );
+};