feat: attend event

Added a new PHP file, logout.php, to handle the logout functionality. The file starts a session, destroys it, and returns a JSON response indicating a successful logout.

Deleted the old logout.php file from the backend/features/authorization directory.

Also added a new PHP file, common.php, to the backend/common directory. This file contains a sendResponse function that sends a JSON response with a success status, an error message, and an optional status code.

Modified the CSS files create-event.css, variables.css, common.css, button.css, and event-list.css to update the styling of various elements.

Modified the JavaScript files create-event.js and register.js to update the backend paths for API requests.

Modified the PHP files create-event.php, login.php, and register.php to update the class names of buttons.

Deleted the old db.php file from the backend/features/db directory.

Modified the JavaScript file login.js to update the backend path for the login API request.

Modified the PHP file event-list.php to add a filter form for events.

Refactored the code to adhere to established coding conventions and improve code readability.