Add startup env validation script

[Goodnessoj]

Summary:

• Closes Add a startup env-validation script that runs config checks before the dev server starts #331
• Adds npm run check-env and runs it before npm run dev via predev.
• Extracts shared app and wallet env validators for the CLI and runtime config.
• Documents the check in README Quick Start and Environment Variables.

Validation:

• npm run check-env -- --file .env.example
• node --check scripts/check-env.js
• node --check lib/config-validation.js
• node --check lib/wallet/validation.js