Add comprehensive unit tests for Security, UserModel, and Validation utilities

- Implemented SecurityTest to validate token generation, CSRF protection, input sanitization, and rate limiting.
- Created UserModelTest to ensure correct database operations for user management, including creation, updating, banning, and fetching active users.
- Developed ValidationTest to verify input validation and sanitization for user IDs, nicknames, messages, and API requests.
- Introduced Security and Validation utility classes with methods for secure token generation, input sanitization, and comprehensive validation rules.

static/css/icons.css

@@ -385,7 +385,6 @@
 /* Interactive button icons - more pronounced effects */
 .icon-button:hover {
     transform: scale(1.15);
-    filter: brightness(1.2);
     transition: all 0.2s cubic-bezier(0.34, 1.56, 0.64, 1);
 }