Fixes full screen toggle not working for boards in new UI

What does this MR do and why?

Fixes full screen toggle not working for boards in new UI

References

• https://gitlab.com/gitlab-org/gitlab/-/issues/567702#note_2746043867
• https://gitlab.com/groups/gitlab-org/-/epics/18930

Screenshots or screen recordings

Before	After
Screen_Recording_2025-09-17_at_9.12.16_PM	Screen_Recording_2025-09-17_at_9.11.02_PM

How to set up and validate locally

1. Enable New UI (i.e. Project Studio)
2. Visit Boards, and try to toggle fullscreen mode.
3. Once it goes full screen (only in this MR branch), view any Issue or Task and verify if it correctly appears in the side panel.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.