A QPixmap snapshot cannot follow palette changes, so expose a QIcon whose
engine resolves the `currentColor` sentinel from the live palette on every
pixmap request. Icons stay legible across light/dark switches without
recreating actions or buttons.
- Resolve the asset, its bytes and its intrinsic size once, and drop them on
ThemeManager::themeChanged/paletteChanged. A repaint does no file I/O and
no theme.cfg parse; the palette color stays part of the render cache key.
- Centre the glyph on QRectF(rect).center(): QRect::center() floors and uses
right = left + width - 1, which shifted every 16x16 item-view glyph up and
left by a logical pixel and clipped its last row and column.
- Take a raster override's intrinsic size from QImageReader instead of the
24x24 SVG default, so a non-square override reports the box it draws.
- Set the device pixel ratio before inserting into the render cache, so a
cache hit no longer detaches the pixmap on every paint.
Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>
