nu doesn't handle resizing intelligently

Describe the bug

It seems that removing smart resize handling (#6016) actually results in an experience different than some terminals, and one that I'd argue is worse.

See, for example, this comparison with fish (fish bottom, nu top):

All I do here is just resize the terminal by dragging the edge -- fish is able to handle this correctly, and keep the right prompt on the right, but nu is not.

This becomes an especially egregious problem when you're using a window manager that always sends at least one resize event on launch n(like Sway). In that case, opening the terminal for the first time always results in an annoying, misplaced right prompt, and for some reason typing a command places the prompt in the middle of the screen, which I'd say makes it nigh-unusable.

How to reproduce

To be very annoyed, use a window manager like Sway, or to be mildly annoyed, just try resizing while having a right prompt.

Expected behavior

Right prompt will stick to the right of the screen, and will be in the correct place initially.

Configuration

Start by tracing how terminal resize events update the right-prompt layout, with particular attention to the behavior removed in #6016. Reproduce the issue by resizing a prompt manually or launching under Sway, then verify that the right prompt stays aligned initially and after resizing without disrupting command entry.