Specifies the order of primary/cancel buttons in dialogs.
This is a platform convention, not visual styling. Different desktop
environments place the confirmation button at different ends of the
button row (Windows/KDE: leading/left, GNOME/macOS/iOS: trailing/right).
It is part of the theme model because “native feel” includes layout
conventions that vary by platform, and it is overridable in theme presets.
Windows uses primary-leftmost per the Microsoft Common Buttons guideline
(Win7) and modern WinUI 3 ContentDialog (“PrimaryButton … Appears as
the leftmost button”). See docs/platform-facts.md:1494, 1513-1520, 1835-1836 for the authoritative citations.