feat: Add ApiErrorDto and ApiValidationErrorDto for standardized error responses
feat: Implement ChangeAvatarRequest, ChangePasswordRequest, ChangeRoleRequest, ChangeStatusRequest, LoginRequest, LoginResponse, and RegisterRequest DTOs
feat: Create custom exceptions for better error handling: BadRequestException, ConflictException, ForbiddenException, NotFoundException, UnauthorizedException, and ValidationException
feat: Add RequestLoggingFilter for logging incoming requests
feat: Implement DevCorsConfig for CORS settings in development environment
feat: Create GlobalExceptionHandler for centralized exception handling
feat: Implement RestAccessDeniedHandler and RestAuthEntryPoint for handling access denied and unauthorized requests
feat: Create CustomPrincipal for Spring Security user details
feat: Implement JwtFilter and JwtService for JWT authentication
feat: Add PasswordGenerator for secure password generation
feat: Implement ServiceAuthFilter for service-level authentication
feat: Create AuthService for handling authentication and password changes
refactor: Update CustomUserDetailsService to use CustomPrincipal
refactor: Update UserService to improve user management and exception handling
fix: Update frontend API base URL in development and production settings
refactor: Clean up Header component and associated CSS
