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() without arg no longer valid; add | undefined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/features/room/MembersDrawer.tsx

@@ -180,9 +180,9 @@ type MembersDrawerProps = {
 export function MembersDrawer({ room, members }: MembersDrawerProps) {
   const mx = useMatrixClient();
   const useAuthentication = useMediaAuthentication();
-  const scrollRef = useRef<HTMLDivElement>(null);
-  const searchInputRef = useRef<HTMLInputElement>(null);
-  const scrollTopAnchorRef = useRef<HTMLDivElement>(null);
+  const scrollRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
+  const searchInputRef = useRef<HTMLInputElement>(null) as React.RefObject<HTMLInputElement>;
+  const scrollTopAnchorRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
   const powerLevels = usePowerLevelsContext();
   const creators = useRoomCreators(room);
   const getPowerTag = useGetMemberPowerTag(room, creators, powerLevels);

src/app/features/room/RoomInput.tsx

@@ -176,7 +176,7 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
       roomUploadAtomFamily,
       selectedFiles.map((f) => f.file),
     );
-    const uploadBoardHandlers = useRef<UploadBoardImperativeHandlers>();
+    const uploadBoardHandlers = useRef<UploadBoardImperativeHandlers | undefined>(undefined);
 
     const imagePackRooms: Room[] = useImagePackRooms(roomId, roomToParents);
 

src/app/features/room/RoomTimeline.tsx

@@ -494,7 +494,7 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
   const imagePackRooms: Room[] = useImagePackRooms(room.roomId, roomToParents);
 
   const [unreadInfo, setUnreadInfo] = useState(() => getRoomUnreadInfo(room, true));
-  const readUptoEventIdRef = useRef<string>();
+  const readUptoEventIdRef = useRef<string | undefined>(undefined);
   if (unreadInfo) {
     readUptoEventIdRef.current = unreadInfo.readUptoEventId;
   }

src/app/features/room/RoomView.tsx

@@ -57,8 +57,8 @@ const shouldFocusMessageField = (evt: KeyboardEvent): boolean => {
 };
 
 export function RoomView({ eventId }: { eventId?: string }) {
-  const roomInputRef = useRef<HTMLDivElement>(null);
-  const roomViewRef = useRef<HTMLDivElement>(null);
+  const roomInputRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
+  const roomViewRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
   const [chatBackground] = useSetting(settingsAtom, 'chatBackground');
   const [lotusTerminal] = useSetting(settingsAtom, 'lotusTerminal');
   const theme = useTheme();