GUI font size not applied on Linux (Qt6 build) despite correct settings.json values #2588

Description

Environment: Ubuntu 22.04.5 LTS, x86_64, HeidiSQL 12.21 (Linux Qt6 .deb build)

Steps to reproduce:

1. Tools > Preferences > General
2. Set GUI font to a named font (e.g. "Noto Sans Mono"), size 5
3. Click OK/Save
4. Fully kill the process (pkill -f heidisql) and relaunch

Expected: GUI renders with the new font/size after restart.

Actual: GUI font/size visually unchanged. Confirmed settings.json correctly contains:

HeidiSQL version and OS

Ubuntu 22.04.5 LTS, x86_64, HeidiSQL 12.21 (Linux Qt6 .deb build)

Database server version

mysql

Reproduction recipe

just try to set as smaller font

Error/Backtrace