Fix Settings tab color mismatch with Settings page background (#19999)

## Summary
The Settings tab color (`SettingsUiTabBrush`) was out of sync with the
Settings page background (`SettingsPageBackground`), causing a visible
mismatch - the tab appeared black while the page was dark gray.

## Changes
Aligned `SettingsUiTabBrush` in `App.xaml` to match
`SettingsPageBackground` from `MainPage.xaml`:

## Validation
- Built and deployed locally as Windows Terminal Dev
- Verified Settings tab and page backgrounds match in Dark and Light
themes
- No impact on other tab colors or theming

Closes #19873
Co-authored-by: Sagar Bhure <sagarbhure@microsoft.com>