Fix settings error text legibility in High Contrast mode (#20098)

Closes #18147

`SystemErrorTextColor` doesn't adapt to High Contrast themes, so the
error details in the "Failed to reload settings" dialog are illegible
(yellow on cream in HC White, for example).

Fix: skip the `ErrorTextBrush` foreground override when High Contrast is
active, letting the text inherit the system HC text color from its
parent element. This matches the existing HC detection pattern used in
`TermControl.cpp` and `MainPage.cpp`.

Validated manually on High Contrast White and Night sky themes.

Co-authored-by: SushaanthSrinivasan <SushaanthSrinivasan@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>