Make the `nav` menu scrollable

[ParadoxV5]

When the <nav> menu has many items (is long) or on short-height windows or screens, the <nav> simply overflows the window with the overflown items cut-off and inaccessible. This quick-fix proposal constraints the <nav> at the bottom and enables scrolling for overflow behaviour.

Although, browsers’ – at least Chrome – scrollbars look thicc and out of place.

I discovered this oversight while trying to fix #48/#62. This can be an initiating step, as the next would be making the bottom retract to make room for the <footer> (which has non-constant height, and I am not aware of how to respond to that).

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[markcmiller86]

@ParadoxV5 thanks for this! It works for me and I really needed it :clap: