fix: The server path is not appended to stylesheets before load in Backoffice (#19192)

* build(deps): bump tiptap from 2.11.5 to 2.11.7

* fix: prepends the system `/css` folder to stylesheets before attempting to load them

* fix: adds more safety around path assumptions

* chore: eslint fix

* fix: prepend only the system path to picked stylesheets

Author: iOvergaard