Prevent iOS Alert and Modal from rendering in the top-left corner

[zoontek]

Summary:

On iOS, both Alert and Modal could render in the top-left corner of the screen instead of filling it.

• RCTAlertController: when the alert window is created from a UIWindowScene, it was initialized without a frame and defaulted to a zero-sized frame. We now set its frame to the scene's coordinate space bounds.
• ModalHostViewUtils: ModalHostViewScreenSize used RCTScreenSize(), whose value is cached once via dispatch_once. If that first read happens before the screen is ready, it caches 0 and stays 0, so the modal renders zero-sized in the top-left corner. We now read RCTViewportSize() on the main queue, which reads the key window's bounds live.

Fixes #45453

Changelog:

[IOS] [FIXED] - Prevent Alert and Modal from rendering in the top-left corner

Test Plan:

• Open an Alert on iOS and confirm it is centered and correctly sized.
• Open a Modal on iOS and confirm it fills the screen instead of appearing in the top-left corner.

Screenshots:

[cortinico]

I never saw this happening tbf. Is this happening consisntely @zoontek ?

[zoontek]

@cortinico No, it's not consistent. It's a timing/race so it's hard to reproduce on demand, but I've hit it myself and got reports from users too (also, see #45453).