Port HiDPI and Scribble fixes to master branch

Fix 25x25 letter markers not showing up for ksudoku_scrible.svg

Correct icon and text positions in game chooser for HiDPI mode

Split out to !23 (merged).

Fix HiDPI rendering in game views

This is cherry-picked from cdaba58a. I exercised the Qt6-based fractional scaling for a bit, and it seems to hold up fine at first glance, phew!

Porting note: While this commit is based on a cherry pick of commit cdaba58a, some parts were substantially rewritten. Improvements include:

• Elimination of generic qApp->devicePixelRatio() calls, instead the dpr is taken directly from the associated scene.
• Better separation between plain rendering and dpr logic.
• Less layout shifts compared to dpr=1.0 upscaling.
• Solved one of the remaining fractional scaling hover remnants.