Feature: Opt-in to viewport-fit=cover

[fpapado]

viewport-fit=cover opts us in to webkit's "full screen" on iPhone X/XS/XR etc.
Safari will not insert automatic padding, and instead it's on us to use the env variables.

Some potential benefits:

• A slightly more spacious landscape view
• A slightly nicer-looking landscape view
• A more correct solution to bottom padding. Again, it seems wrong that Safari will report "0" for bottom padding if not opting in to full-screen, especially when it is always present

Things to check:

• Left/right padding on container should be 1rem/calc(inset + 1rem)/max(inset, 1rem), enhancing accordingly
• Same for header title + nav left/right padding
• Container top padding should be header/calc(header+inset)
• Container bottom padding should be ???
• Header nav bottom padding should be 0.5rem/calc(inset-bottom+0.5rem)/max(inset-bottom, 0.5rem)

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[fpapado]

Seems ok on browserstack, though real use may differ. Merging to get better data points in practice.