fix: rework compact nav and centralized back routing

Use master-detail layout on skinny windows so server and channel lists share one screen, with back returning to the space list instead of redirect loops. Add useBackRoute for title-bar and page-header back buttons with view transitions.

src/app/pages/client/space/SpaceIndexRedirect.tsx

@@ -5,17 +5,24 @@ import { useSpace } from '../../../hooks/useSpace';
 import { shouldShowForumLobby } from '../../../utils/room';
 import { getSpaceDefaultPath } from '../../pathUtils';
 import { ForumFeedPage } from '../../../features/forum/ForumFeedPage';
+import { useCompactNav } from '../../../hooks/useCompactNav';
 
-/** Sends /:spaceId/ to the forum feed or space lobby. */
+/** Sends /:spaceId/ to the forum feed or space lobby (desktop). On compact nav, stay on index for the channel list. */
 export function SpaceIndexRedirect() {
   const mx = useMatrixClient();
   const space = useSpace();
   const navigate = useNavigate();
+  const compact = useCompactNav();
   const isForum = shouldShowForumLobby(space);
 
   useEffect(() => {
+    if (compact) return;
     navigate(getSpaceDefaultPath(mx, space.roomId), { replace: true });
-  }, [mx, navigate, space.roomId]);
+  }, [compact, mx, navigate, space.roomId]);
+
+  if (compact) {
+    return null;
+  }
 
   // Avoid a blank main pane while redirecting forum spaces to /feed/.
   if (isForum) {