fix: FilesExplorerForm icons for differing browser text sizes

[jayschwa]

I run my browser with a non-standard text size and I noticed that the FilesExplorerForm icons do not look correct. This changes the icon heights to relative units from the absolute (default) pixels.

Very small and very large text sizes before this change:

After:

I am using the Dark Reader extension for the dark theme. It is not part of this PR.

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[lidel]

Thanks!

I guess there is no harm in merging this (fwiw I see no regression with default font size)