net-im/nheko: update to 0.12.1, latest upstream

Release-notes are at
https://github.com/Nheko-Reborn/nheko/releases/tag/v0.12.1

There's a couple of encoding-bugs (labeled "security") in this
release, as well as a bunch of general improvements. It's still
a release from August 2025, so the patch (in FreeBSD ports, picked
from a merged-fix upstream) for Qt 6.10 compatibility remains.

Reported-By: makc@