Add color customization support to the Config struct with 13 new char[16] fields covering panel background/foreground, workspace active/inactive/urgent, title focused/unfocused background/foreground, border focused/unfocused, and notification background/foreground. Implement default value initialization in config_set_defaults using strncpy with DEFAULT_* constants, and add parsing logic in handle_config_entry for the new "colors" section to populate these fields from configuration files.