fix Cannot create a SharedArrayBuffer problem

[Indhiraraj]

Fixes #2094

Changes:

1. Added Security Headers:
   Implemented 'Cross-Origin-Embedder-Policy': 'require-corp' and 'Cross-Origin-Opener-Policy': 'same-origin' in both server.js and previewServer.js to enhance cross-origin security and enable origin isolation.
2. Configured Proxy:
   Created a proxy middleware to handle requests to the preview server, enabling communication despite the strict cross-origin policies.
3. New Environment Variable:
   Introduced a new environment variable PREVIEW_SERVER_URL (e.g., localhost:8000/preview) that points to the proxy for preview server requests.
4. Updated Client-Side Code:
   Replaced all references to PREVIEW_URL in the client code with the newly introduced PREVIEW_SERVER_URL, ensuring compatibility with the proxy setup.
5. Updated Environment Example File:
   Added the new PREVIEW_SERVER_URL variable to the .env.example file to maintain consistency and support for future setups.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123