fix: `<img>` aspect ratio w/ `height: auto`

[Pinjasaur]

Complement the max-width: 100% with a height: auto to preserve <img> aspect ratios, particularly when it's not wrapped in a block parent e.g. <p> or <figure> most commonly.

Related: https://www.smashingmagazine.com/2020/03/setting-height-width-images-important-again/

An example scenario that this fixes: https://reader.miniflux.app/share/8bd2c1ae6b6577ef9c81106964f9a9e9865edb0d

Before:

After:

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