loadPixmap() rendered all SVGs at a hardcoded 64x64 pixels regardless of
display DPI. On HiDPI screens, Qt then downscaled these low-resolution
pixmaps to the toolbar icon size, producing chunky/aliased icons.
Multiply the render size by getMaximumDPR() and tag the resulting pixmap
with the correct devicePixelRatio, matching the pattern already used in
pixmapFromSvg(const char* name, QSizeF size). This ensures SVGs are
rasterized at the physical resolution needed for crisp display.
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