fix: conform to new frontend-base CSS ownership model

Per the updated frontend-base guidance, the composing site owns global
styles and apps manage their own. Imports the shell style manifest
(@openedx/frontend-base/shell/style) and brand CSS directly from the
site.config files, removing the intermediate site.scss.

Uses @openedx/brand-openedx directly (no need for an alias since
package.json and the site configs are themselves configuration), and
bumps it to v2.

Also adds notificationsApp to the site configs.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: arbrandes