Deploy to Vercel with Environment Variables

Help me prepare this app for deployment to Vercel. First, create a .env.example file that lists all required environment variables with placeholder values and a comment explaining what each one is used for. Then update the code to read all sensitive configuration (API keys, database URLs, auth secrets) from environment variables rather than hardcoded values. Add a check on app startup that throws a clear error if any required environment variable is missing, listing exactly which ones are absent. Finally, create a vercel.json config file with the correct build command, output directory, and any necessary rewrites for client-side routing. The deployment should work with zero manual configuration on Vercel's dashboard beyond pasting the environment variables.