Add real fullscreen mode using the Fullscreen API (like YouTube videos)

**Context**

Currently, the BookReader fullscreen button only maximizes the viewer inside the browser tab.
It would be great to have true fullscreen support (like YouTube videos) using the Fullscreen API, where the browser UI is hidden and only the book pages are visible.

This would improve reading immersion.

**Proposal & Constraints**

* Add a true fullscreen mode to BookReader using the Fullscreen API (element.requestFullscreen()

**Precedent:**

* YouTube, Dailymotion and many web-based document/image viewers already use the Fullscreen API successfully.

* The Fullscreen API is well-supported across all modern browsers, including mobile.

**Additional Resources**

Example implementation: element.requestFullscreen()
https://developer.mozilla.org/en-US/docs/Web/API/Fullscreen_API

**Success Metrics**

* Increase in time spent per reading session (less distraction → more immersion)

* Positive user feedback about fullscreen mode

**Stakeholders**

@cdrini

Start by locating the BookReader fullscreen button and the existing viewer-maximization behavior, then review the linked MDN Fullscreen API documentation. Done means the BookReader can request true browser fullscreen so the book pages are visible without browser UI, with behavior checked in supported modern browsers.

Written by the indexing model from the issue text.