refactor: replace @vitejs/plugin-react-swc with @vitejs/plugin-react and update Vite config paths

- Updated package.json files in demo and lib packages to use @vitejs/plugin-react version 5.1.1 instead of @vitejs/plugin-react-swc version 4.3.1.
- Modified vite.config.ts files in demo and lib packages to resolve paths using import.meta.dirname for better directory handling.

Author: freds-dev

chartlets.js/CHANGES.md

@@ -1,5 +1,8 @@
 ## Version 0.2.2 (in development)
 
+* Replaced `@vitejs/plugin-react-swc` with `@vitejs/plugin-react` and
+  updated Vite configs to use `import.meta.dirname`.
+
 ## Version 0.2.1 (from 2026/08/11)
 
 * Updated dependencies