Update to React 19

Fixes https://github.com/nomcopter/react-mosaic/issues/225 Changes proposed in this pull request: Expand peerDependencies to support React 19 and ReactDOM 19. Verify that the library builds and typechecks cleanly with React 19 installed. No runtime or production code changes — this is strictly a compatibility update. Clarify development behavior when running yarn start with React 19: The demo shows a runtime error overlay originating from Blueprint.js, which does not officially support React 19. The error can be dismissed and the demo continues to function normally. Blueprint.js is used only in the demo app and is not included in the published package, so this does not affect consumers of the library. Reviewers should focus on: peerDependencies: Ensure the updated React/ReactDOM version ranges remain compatible with React 16–18 users. Production bundle: Confirm Blueprint.js is not part of the distributed build. Demo behavior: Validate that the error shown in development is limited to the demo overlay and does not break mosaic functionality. Types & exports: Ensure no changes to public API surface or TypeScript declarations. <img width="1201" height="859" alt="image" src="https://github.com/user-attachments/assets/9d71d634-c941-4106-b439-0cb38617f897" />