5K and 6K displays read correctly in Display Diagnostics

A focused fix for the Pro Display Diagnostics screen.

What's fixed

5K and 6K displays no longer show as 4K in Display Diagnostics. WhatCable already reads your display's true mode straight from macOS, but that reading was only reaching the command-line output, not the Display Diagnostics screen in the app. The screen now uses the same live reading, so an Apple Studio Display reads 5120 × 2880 instead of falling back to the 4K value its spec sheet reports. Thanks to @cannotcollide for the report and for sticking with it on issue #249.

The "running at full quality" verdict was always correct; only the resolution label was wrong.

brew upgrade --cask whatcable, download from Releases, or update in the app.