React 19 compatibility

#### Fixes #217 #### Changes proposed in this pull request: * Update `rdndmb-html5-to-touch` and `react-dnd-multi-backend` to support React 19 * Replace `JSX.Element` with `ReactElement` and update some `ref`s due to React 19 type changes #### Reviewers should focus on: <!-- fill this out --> #### Screenshot <!-- include an image of the most relevant user-facing change, if any -->