Fix fractional DPI drawing issues with popup menus

[Copilot]

At fractional DPI scales (e.g. 150%), popup menu borders have missing corners and selected item backgrounds show extra pixel gaps. Root cause: the device→logical→device pixel round-trip in MenuDropDown.Show() uses Math.Round, which can shrink the rendering buffer by 1px, clipping border lines and misaligning item fills.

• Show(): Use Math.Ceiling instead of Math.Round when converting device pixel dimensions to logical pixels for the popup size, ensuring the buffer is never smaller than the calculated menu dimensions

// Before
popup.Size = ScaleSize(new Size(width, height), 1 / (float)Scaling, 1 / (float)Scaling);

// After
popup.Size = new Size(
    (int)Math.Ceiling(width / Scaling),
    (int)Math.Ceiling(height / Scaling));

• LayoutItems(): Use Math.Max(calculated, ScaledSize) so items expand to fill any extra pixels introduced by ceiling rounding, preventing background-color gaps between items and borders

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[jpobst]

@copilot This did not seem to fix any of the issues, updated screenshot attached.