Security fixes and logging

• Implemented optional audit logging system to allow tracking of all impersonation events for security monitoring, compliance, and forensic analysis.
• Added session regeneration to prevent session fixation attacks when starting or stopping impersonation.
• Implemented validation of user identifiers.
• Prevented XSS in HTML attribute context and improved type safety.
• Fixed XSS vulnerability in user selector.

Initial release.

v1.0.0

Update PHPStan level in README.