1-frame hover delay on newly-spawned entities

## Bevy version

0.16.0

## What you did

Spawn a UI node directly under the user's cursor that changes its background color based on interaction state.

## What went wrong

The frame that it spawns in, it has `Interaction::None` despite being under the user's cursor, so it flashes that color for 1 frame and then becomes `Interaction::Hover` the next frame.

Demonstration with colors changed for clarity:

https://github.com/user-attachments/assets/ad9785f0-7980-46e7-a38e-69cb569b1307

This seems to be because `ui_focus_system` iterates over `UiStack` to find hovered entities, but `UiStack` is not updated for the frame until `ui_stack_system` runs in `PostUpdate`.