chore: upgrade React 18→19 and fix breaking type changes

- react 18.2.0 to 19.2.6
- react-dom 18.2.0 to 19.2.6
- @types/react 18.2.39 to 19.2.15
- @types/react-dom 18.2.17 to 19.2.3

React 19 breaking changes fixed:
- useRef<T>(null) now returns RefObject<T | null>; cast to
  RefObject<T> at 16 component call sites (safe, runtime unchanged)
- useRef<T>() without arg no longer valid; add | undefined>(undefined)
  in useDebounce, useFileDrop, useThrottle, useVirtualPaginator hooks,
  RoomInput, RoomTimeline, and ClientNonUIFeatures
- useReducer<typeof reducer> 1-arg form removed; drop explicit type arg
  in useForceUpdate (inferred from reducer function)
- global JSX namespace removed; import type { JSX } from react in
  react-custom-html-parser.tsx

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/pages/client/ClientNonUIFeatures.tsx

@@ -130,7 +130,7 @@ function InviteNotifications() {
 
 function MessageNotifications() {
   const audioRef = useRef<HTMLAudioElement>(null);
-  const notifRef = useRef<Notification>();
+  const notifRef = useRef<Notification | undefined>(undefined);
   const unreadCacheRef = useRef<Map<string, UnreadInfo>>(new Map());
   const mx = useMatrixClient();
   const useAuthentication = useMediaAuthentication();

src/app/pages/client/SidebarNav.tsx

@@ -20,7 +20,7 @@ import {
 import { CreateTab } from './sidebar/CreateTab';
 
 export function SidebarNav() {
-  const scrollRef = useRef<HTMLDivElement>(null);
+  const scrollRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
 
   return (
     <Sidebar>

src/app/pages/client/explore/Server.tsx

@@ -355,8 +355,8 @@ export function PublicRooms() {
   const [searchParams] = useSearchParams();
   const serverSearchParams = useServerSearchParams(searchParams);
   const isSearch = !!serverSearchParams.term;
-  const scrollRef = useRef<HTMLDivElement>(null);
-  const searchInputRef = useRef<HTMLInputElement>(null);
+  const scrollRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
+  const searchInputRef = useRef<HTMLInputElement>(null) as React.RefObject<HTMLInputElement>;
   const navigate = useNavigate();
   const roomTypeFilters = useRoomTypeFilters();
 

src/app/pages/client/home/Search.tsx

@@ -7,7 +7,7 @@ import { ScreenSize, useScreenSizeContext } from '../../../hooks/useScreenSize';
 import { BackRouteHandler } from '../../../components/BackRouteHandler';
 
 export function HomeSearch() {
-  const scrollRef = useRef<HTMLDivElement>(null);
+  const scrollRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
   const rooms = useHomeRooms();
   const screenSize = useScreenSizeContext();
 

src/app/pages/client/inbox/Notifications.tsx

@@ -575,8 +575,8 @@ export function Notifications() {
   const { navigateRoom } = useRoomNavigate();
   const [searchParams, setSearchParams] = useSearchParams();
   const notificationsSearchParams = useNotificationsSearchParams(searchParams);
-  const scrollRef = useRef<HTMLDivElement>(null);
-  const scrollTopAnchorRef = useRef<HTMLDivElement>(null);
+  const scrollRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
+  const scrollTopAnchorRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
   const [refreshIntervalTime, setRefreshIntervalTime] = useState(DEFAULT_REFRESH_MS);
 
   const onlyHighlight = notificationsSearchParams.only === 'highlight';

src/app/pages/client/sidebar/SpaceTabs.tsx

@@ -402,7 +402,7 @@ function SpaceTab({
 }: SpaceTabProps) {
   const mx = useMatrixClient();
   const useAuthentication = useMediaAuthentication();
-  const targetRef = useRef<HTMLDivElement>(null);
+  const targetRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
 
   const spaceDraggable: SidebarDraggable = useMemo(
     () =>
@@ -506,8 +506,8 @@ type OpenedSpaceFolderProps = {
   children?: ReactNode;
 };
 function OpenedSpaceFolder({ folder, onClose, children }: OpenedSpaceFolderProps) {
-  const aboveTargetRef = useRef<HTMLDivElement>(null);
-  const belowTargetRef = useRef<HTMLDivElement>(null);
+  const aboveTargetRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
+  const belowTargetRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
 
   const spaceDraggable: SidebarDraggable = useMemo(() => ({ folder, open: true }), [folder]);
 
@@ -554,7 +554,7 @@ function ClosedSpaceFolder({
 }: ClosedSpaceFolderProps) {
   const mx = useMatrixClient();
   const useAuthentication = useMediaAuthentication();
-  const handlerRef = useRef<HTMLDivElement>(null);
+  const handlerRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
 
   const spaceDraggable: FolderDraggable = useMemo(() => ({ folder }), [folder]);
   useDraggableItem(spaceDraggable, handlerRef, onDragging);

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

@@ -14,7 +14,7 @@ import { BackRouteHandler } from '../../../components/BackRouteHandler';
 
 export function SpaceSearch() {
   const mx = useMatrixClient();
-  const scrollRef = useRef<HTMLDivElement>(null);
+  const scrollRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
   const space = useSpace();
   const screenSize = useScreenSizeContext();