Page action layout broken when using non-default DPI settings

[simu]

When running Firefox on Linux with a non-default system-wide DPI setting (144dpi in my case), the layout of the page action does not reflow nicely in the page action popup.

A likely culprit is the fixed width styling for the HTML elements of the popup: https://github.com/simu/passwordhasherplus/blob/b9c7f7cccb742028af4760d33654d1f4e803164b/popup.css#L1-L85

These should be rewritten using CSS which handles different DPI settings more gracefully.