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() without arg no longer valid; add | undefined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/lobby/Lobby.tsx

@@ -159,8 +159,8 @@ export function Lobby() {
   const lex = useMemo(() => new ASCIILexicalTable(' '.charCodeAt(0), '~'.charCodeAt(0), 6), []);
   const members = useRoomMembers(mx, space.roomId);
 
-  const scrollRef = useRef<HTMLDivElement>(null);
-  const heroSectionRef = useRef<HTMLDivElement>(null);
+  const scrollRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
+  const heroSectionRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
   const [heroSectionHeight, setHeroSectionHeight] = useState<number>();
   const [spaceRooms, setSpaceRooms] = useAtom(spaceRoomsAtom);
   const [isDrawer] = useSetting(settingsAtom, 'isPeopleDrawer');

src/app/features/lobby/RoomItem.tsx

@@ -315,8 +315,8 @@ export const RoomItemCard = as<'div', RoomItemCardProps>(
     const useAuthentication = useMediaAuthentication();
     const { roomId, content } = item;
     const room = getRoom(roomId);
-    const targetRef = useRef<HTMLDivElement>(null);
-    const targetHandleRef = useRef<HTMLDivElement>(null);
+    const targetRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
+    const targetHandleRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
     useDraggableItem(item, targetRef, onDragging, targetHandleRef);
 
     const joined = room?.getMyMembership() === Membership.Join;

src/app/features/lobby/SpaceItem.tsx

@@ -429,7 +429,7 @@ export const SpaceItemCard = as<'div', SpaceItemCardProps>(
     const useAuthentication = useMediaAuthentication();
     const { roomId, content } = item;
     const space = getRoom(roomId);
-    const targetRef = useRef<HTMLDivElement>(null);
+    const targetRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
     useDraggableItem(item, targetRef, onDragging);
 
     return (