Add user authentication system

- Implement secure user registration and login
- Add password hashing with PBKDF2 and random salts
- Create session-based authentication with secure tokens
- Support user deactivation and session management
- Include comprehensive unit tests for authentication
- Integrate authentication demo into main application

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

main.py

@@ -6,6 +6,7 @@ Main application entry point for the sandbox project.
 import json
 import os
 from datetime import datetime
+from src.auth import UserManager
 
 def load_config():
     """Load configuration from config.json"""
@@ -24,6 +25,27 @@ def main():
     
     if config.get("debug"):
         print("Running in debug mode")
+    
+    # Initialize authentication system
+    auth_manager = UserManager()
+    print("\n--- Authentication System Demo ---")
+    
+    try:
+        # Register a demo user
+        auth_manager.register_user("demo_user", "demo@example.com", "secure_password123")
+        print("✓ Demo user registered successfully")
+        
+        # Authenticate the user
+        session_token = auth_manager.authenticate("demo_user", "secure_password123")
+        print("✓ User authenticated successfully")
+        print(f"Session token: {session_token[:20]}...")
+        
+        # Validate session
+        username = auth_manager.validate_session(session_token)
+        print(f"✓ Session validated for user: {username}")
+        
+    except Exception as e:
+        print(f"✗ Authentication error: {e}")
 
 if __name__ == "__main__":
     main()