♻️ refactor: remove redundant credential module

- Delete src/credential.rs - module was unused in actual codebase
- Remove credential module references from lib.rs exports
- Update documentation to use standard OAuth2 terminology
- Fix CLI docs to reference correct credential_file config
- Application uses ConsoleApplicationSecret directly instead

The custom Credential struct duplicated functionality already
provided by yup_oauth2::ConsoleApplicationSecret. All credential
loading is handled by ClientConfig::new_from_configuration() which
uses the standard OAuth2 types.

docs/lib.md

@@ -304,15 +304,15 @@ Log levels:
 ### Required Scopes
 The library requires the `https://mail.google.com/` scope for full Gmail access.
 
-### Credential File Security
-- Store credential files securely (not in version control)
+### OAuth2 File Security
+- Store OAuth2 credential files securely (not in version control)
 - Use restrictive file permissions (600)
 - Consider using environment variables in production
 
 ## Troubleshooting
 
 ### Authentication Issues
-1. Verify credential file path and format
+1. Verify OAuth2 credential file path and JSON format
 2. Check OAuth2 client is configured for "Desktop Application"
 3. Ensure redirect URI matches configuration
 4. Clear token cache: `rm -rf ~/.cull-gmail/gmail1`