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/Header.tsx

@@ -6,8 +6,13 @@ Please see LICENSE in the repository root for full details.
 */
 
 import classNames from "classnames";
-import { type Ref, type FC, type HTMLAttributes, type ReactNode } from "react";
-import { Link } from "react-router-dom";
+import {
+  type Ref,
+  type FC,
+  type HTMLAttributes,
+  type ReactNode,
+  useCallback,
+} from "react";
 import { useTranslation } from "react-i18next";
 import { Heading, Text } from "@vector-im/compound-web";
 import { UserProfileIcon } from "@vector-im/compound-design-tokens/assets/web/icons";
@@ -17,6 +22,7 @@ import Logo from "./icons/Logo.svg?react";
 import { Avatar, Size } from "./Avatar";
 import { EncryptionLock } from "./room/EncryptionLock";
 import { useRootSizeMatches } from "./useRootSize";
+import { useLeaveToHome } from "./LeaveToHomeContext";
 import { DisconnectedBanner } from "./DisconnectedBanner";
 
 interface HeaderProps extends HTMLAttributes<HTMLElement> {
@@ -112,17 +118,30 @@ interface HeaderLogoProps {
   className?: string;
 }
 
+/**
+ * The logo, which is also the way home — when there is a home to go to. As a
+ * component there is not, and it is just the logo.
+ */
 export const HeaderLogo: FC<HeaderLogoProps> = ({ className }) => {
   const { t } = useTranslation();
+  const leaveToHome = useLeaveToHome();
+  const onClick = useCallback(() => leaveToHome?.(), [leaveToHome]);
 
+  if (leaveToHome === null)
+    return (
+      <div className={classNames(styles.headerLogo, className)}>
+        <Logo />
+      </div>
+    );
   return (
-    <Link
+    <button
+      type="button"
       className={classNames(styles.headerLogo, className)}
-      to="/"
+      onClick={onClick}
       aria-label={t("header_label")}
     >
       <Logo />
-    </Link>
+    </button>
   );
 };