fix: show active speaker in full screen when no participant is selected

As well as fixing the issue of updating the focused participant to the latest speaker, this MR also (re-)introduced pinning for the full-screen view.

If a user activates full-screen mode by clicking the button on a grid tile, the selected participant will be pinned by default.

Closes https://git.opentalk.dev/opentalk/frontend/web/web-app/-/issues/2719