Embeddable public text preview page

Hi there 👋,

This is a feature request to be able to have a similar feature as the NC calendar app and be able to iframe embed a public file.

Indeed, the Text application can display beautiful markdown previews and thus a file can be shared with a public URL such as https://nextcloud.example.org/s/filePublicId.

Would it be possible to have an embeddable preview available at the URL (for instance) https://nextcloud.example.org/s/filePublicId/embed?

Similarly to the Calendar app (https://nextcloud.example.org/apps/calendar/embed/publicId) the page would allow embedding via an iframe on other domains and remove NC's header and footer.

This implies:

• Adding a CSP policy (similarly to the Calendar app) in a new dedicated embeddable route in the backend
• Making sure to remove NC header and footer on the embedded preview

What do you think?

Start by reading the existing public Text file preview flow and the Calendar app's lib/Controller/PublicViewController.php example linked in the issue. Trace the /s/filePublicId route, then define the dedicated embed route and its page behavior; done means a public preview can load in an iframe, uses an appropriate CSP, and omits the Nextcloud header and footer.