Refactor environment variable handling for Cloudflare Workers

Commit notes

- Updated .dev.vars to remove VITE_ prefix and include API_URL and SITE_URL for local development. - Created CLOUDFLARE_ENV_VARS.md to document the new environment variable structure and usage. - Modified wrangler.jsonc to define production and preview environment variables. - Refactored loaders and API utilities to utilize the new environment variable system. - Enhanced React components to access environment variables through loaders, ensuring secure handling of sensitive data.