fix: Improve env file security and remove unused config

- Remove unused DENSER_SERVER_COOKIE_NAME_PREFIX (cookie prefix
  comes from REACT_APP_APP_NAME, not this variable)
- Add comments: secrets must be unique per app for defense in depth
- Remove chat integration from wallet (minimizes attack surface for
  active key operations)
- Add .env.blog and .env.wallet to .gitignore to prevent accidental
  secret commits