fix: ui: correct loading screen rectangle dimensions

[lorenzoberts]

This PR fixes a bug where the loading screen message doesn't appear in smaller terminal sizes. The idea behind the solution is to consider both the text size and frame dimensions to calculate the desired rectangle percentage. I've also added an automated test for this calculation.

Closes #75

Screenshots after the solution: Really small length, really small width: Small length, small width: Small length, big width: Small width, big length: Big sized terminal: Full screen terminal:

If there is no length and width to show the message there is nothing we can do:

[davidbtadokoro]

Hey @lorenzoberts. Really, really neat fix!! Thanks for the great explanation and print screens you've added to the PR message, as they help me validate the change, as well as give the PR more context. You've not only fixed the bug but also made the loading screen more dynamic and adaptable to the terminal, so kudos!

Cleanly applied into the unstable branch :+1: