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/room/LobbyView.test.tsx

@@ -7,7 +7,7 @@ Please see LICENSE in the repository root for full details.
 
 import { describe, expect, it, vi } from "vitest";
 import { render } from "@testing-library/react";
-import { BrowserRouter } from "react-router-dom";
+import { LeaveToHomeProvider } from "../LeaveToHomeContext";
 import { TooltipProvider } from "@vector-im/compound-web";
 import { type MatrixClient } from "matrix-js-sdk";
 import { axe } from "vitest-axe";
@@ -26,6 +26,9 @@ import lobbyStyles from "./LobbyView.module.css";
 import headerStyles from "../Header.module.css";
 import { AppBar } from "../AppBar";
 
+// Somewhere to go home to, so that the lobby offers the way back
+const leaveToHome = vi.fn();
+
 vi.mock("@livekit/components-react", () => ({
   usePreviewTracks: (): unknown[] => [],
 }));
@@ -93,14 +96,14 @@ function renderLobbyView(
     />
   );
   return render(
-    <BrowserRouter>
+    <LeaveToHomeProvider value={leaveToHome}>
       <MediaDevicesContext value={mediaDevices}>
         <TooltipProvider>
           {withAppBar && <AppBar>{lobbyView}</AppBar>}
           {!withAppBar && lobbyView}
         </TooltipProvider>
       </MediaDevicesContext>
-    </BrowserRouter>,
+    </LeaveToHomeProvider>,
   );
 }