Add way to differentiate settings between desktop and mobile layout

[BrokenEagle]

Discussed a bit on the Danbooru forum, and related to issues #3067 and #3073.

Basically, the "Fit images to window" user setting is very convenient when viewing posts from a mobile device, but not so much on a desktop with the luxury of additional screen space. I'm not sure how many other user settings would be beneficial to differentiate between the two layouts. Thoughts?

[stale]

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