feat: automatic DPI detection

[fdcote]

This PR implements automatic DPI detection, ensuring that zoom percentage (after being entered with the zoom command) is always relative to the actual physical size of the content, regardless of which display the window is on.

Note: Due to the variability of display reporting in Linux, this feature is limited to macOS for now, until a general Linux solution is found.

[freref]

I'm not sure this is working as expected for me, whats the desired behavior when I set zoom to 100%? The picture below shows 100% zoom on my machine.

[fdcote]

At 100% zoom, the document should appear on screen at its actual physical size (letter, A4, etc.). In other words, the output should match the size of a printed copy held up to the screen. The preview app in macOS adopts this behavior, so it's also possible to verify the expected size that way (at the same zoom level).

[freref]

Ok works for me, small nitpick: is there a reason we calculate the dpi every time we call setZoom as opposed to once when loading the config?

[fdcote]

Yes. The reason is that the DPI could change if the window is moved to another display (in a multi-display setup).

[freref]

Makes sense, lovely work as always!