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() without arg no longer valid; add | undefined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/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);