AppTypography.dark constructor
The dark-theme typography scale.
Identical to AppTypography.light today — this app's text styles have never varied by brightness (colour, not weight or size, carries the light/dark distinction here). Kept as its own factory, rather than reusing AppTypography.light directly at call sites, so a future dark-specific adjustment (e.g. optical weight compensation) has a single place to land without touching AppTheme.
Implementation
factory AppTypography.dark() => _build();