Respect browser font size instead defining a fixed px size

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Is your feature request related to a problem? Please describe.
Currently Nextcloud ignores the browser setting for the primary font size and defines a default font size of 15px. This means, nevertheless, which default font size I set as a user, all apps base on 15px font size.

Out of accessibility reasons I wonder, if it would be a good decision to get away from fixed px definitions to a relative font size by using rem and em.

Describe the solution you'd like

• Define a default font-size of 16px as used aqs the default by most of the browsers.
• Change font size definitions from px to rem
• Change any width (and maybe heights) to be calculated from the font size
• Same for margins, paddings and other elements like borders
• Define media queries by em (or rem, if safari has no more issues with that)
• Adopt this to nextcloud-vue and other libs

Additional context
This means some good planning and communication, since apps have to follow this change.

Start by reviewing how the current 15px font size and other px-based dimensions are defined across Nextcloud and nextcloud-vue, then assess the browser and Safari media-query considerations mentioned. Done means a coordinated, documented migration to relative font sizing and related dimensions across the affected apps and libraries, with responsive behavior preserved.