BookStack 26.03: new theme module system & enhanced API for developers

BookStack 26.03 introduces a theme module system, letting multiple modules coexist within themes, and adds a new install command for simplified module management. API responses for reading books now include data on parent shelves to streamline data retrieval for developers. The update also brings new logical theme events, making it easier to customize rendered content and extend page logic, and UI refinements, updated translations, and minor fixes, while strengthening security and content filtering options.