popups.css: make "native link dialog" adaptive (MR 44)

Based on user0's work, I've implemented a slightly different fix for
this menu:

* max-width only, because that's enough to fix the menu
* 100% instead of 100vw, as that's less in most cases, but enough.

Related: https://codeberg.org/user0/mobile-config-firefox/src/commit/f2381fd45eeef10ffb173a56f8106e6d68b981be/src/userChrome/06_popups.css#L85-L89
Fixes: #65