Add screencast-only fullscreen mode for group call screenshare

[paterkleomenis]

Summary

• add a dedicated fullscreen toggle button next to mute in wide group-call controls for active screenshare
• add screencast-only fullscreen mode that hides the members panel and expands the screenshare viewport
• preserve and align double-click screenshare fullscreen behavior with the new screencast-only state
• auto-hide center controls on inactivity in screencast-only fullscreen and show them again on mouse movement
• disable speaking-outline rendering in fullscreen to avoid full-frame green highlighting artifacts

Testing

• cmake --build out --config Debug --target Telegram

[ilya-fedin]

Is it intended that this commit is also present in #30278?

[paterkleomenis]

No, is it an issue?> Is it intended that this commit is also present in #30278?

[ilya-fedin]

No, is it an issue?

Yes, if that other PR will get merged, this one will conflict.

[paterkleomenis]

No, is it an issue?

Yes, if that other PR will get merged, this one will conflict.

I fixed it

[ilya-fedin]

I noticed two problems:

1. The number of participants duplicates and the duplicate one is even visible on top of the stream:
2. Both the new screenFull button and the maximize button on titlebar lead to fullscreen. I assume it's not intended? Perhaps the new button is not needed if you want fullscreen inside titlebar like in one-to-one calls.

[paterkleomenis]

Fixed both issues:

1. Participant count duplication — the _subtitle widget (showing participant count) could persist on top of the stream when transitioning to screencast-only fullscreen. Now it's explicitly destroyed when entering that mode.

2. Redundant screenFull button — removed the separate fullscreen toggle button entirely. The titlebar maximize button already handles fullscreen for screenshare, same as in one-to-one calls.