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>

Author: sagarbhure-msft

src/cascadia/TerminalApp/App.xaml

@@ -192,7 +192,7 @@
                                             ResourceKey="TabViewBackground" />
 
                             <SolidColorBrush x:Key="SettingsUiTabBrush"
-                                             Color="#0c0c0c" />
+                                             Color="#282828" />
 
                             <SolidColorBrush x:Key="BroadcastPaneBorderColor"
                                              Color="{StaticResource SystemAccentColorDark2}" />
@@ -211,7 +211,7 @@
                                             ResourceKey="TabViewBackground" />
 
                             <SolidColorBrush x:Key="SettingsUiTabBrush"
-                                             Color="#ffffff" />
+                                             Color="#F9F9F9" />
 
                             <SolidColorBrush x:Key="BroadcastPaneBorderColor"
                                              Color="{StaticResource SystemAccentColorLight2}" />
@@ -234,7 +234,7 @@
                                             ResourceKey="SystemColorButtonFaceColorBrush" />
 
                             <StaticResource x:Key="SettingsUiTabBrush"
-                                            ResourceKey="SystemControlBackgroundBaseLowBrush" />
+                                            ResourceKey="SystemColorWindowBrush" />
 
                             <SolidColorBrush x:Key="BroadcastPaneBorderColor"
                                              Color="{StaticResource SystemColorHighlightColor}" />