fix(settings-ui): restore General page navigation (#49801)

## Summary of the Pull Request

Fixes Settings navigation to the General page by applying the correctly
typed `SecondaryIsEnabledTextBlockStyle` to
`GeneralPage_UpdateChannelDescription` in
`src/settings-ui/Settings.UI/SettingsXAML/Views/GeneralPage.xaml`.

The previous `SecondaryTextStyle` targets `TextBlock`, so WinUI threw a
`XamlParseException` when applying it to the custom `IsEnabledTextBlock`
control. This prevented `GeneralPage.InitializeComponent()` from
completing.

## PR Checklist

- [x] **Communication:** Regression identified while validating the
v0.101 preview candidate
- [x] **Tests:** Existing Settings tests pass and the runtime regression
was verified
- [x] **Localization:** No end-user-facing strings changed

## Detailed Description of the Pull Request / Additional comments

PR #49722 introduced the incompatible style assignment. This change
reuses the existing `SecondaryIsEnabledTextBlockStyle`, which targets
`controls:IsEnabledTextBlock` and preserves the intended secondary
foreground and 12px font size.

The same regression is present on `stable`; this fix should be included
in the v0.101 release branch after merge.

## Validation Steps Performed

- Built `src/settings-ui/PowerToys.Settings.slnf` in Release x64.
- Ran `Settings.UI.UnitTests.dll`: 185 passed, 0 failed, 0 skipped.
- Launched the fixed Release Settings binary directly on `Overview`; the
General header and Update channel controls rendered.
- Confirmed no new `NavigationService` or `XamlParseException` log
entries and no relevant Event Viewer crash.

Copilot-Session: e9f79ac2-9a7b-4083-834c-0d87e8c83bfd

src/settings-ui/Settings.UI/SettingsXAML/Views/GeneralPage.xaml

@@ -108,7 +108,7 @@
                             IsEnabled="{x:Bind ViewModel.IsIncludePrereleaseUpdatesCardEnabled, Mode=OneWay}">
                             <tkcontrols:SettingsExpander.Description>
                                 <StackPanel Orientation="Vertical">
-                                    <ptcontrols:IsEnabledTextBlock x:Uid="GeneralPage_UpdateChannelDescription" Style="{StaticResource SecondaryTextStyle}" />
+                                    <ptcontrols:IsEnabledTextBlock x:Uid="GeneralPage_UpdateChannelDescription" Style="{StaticResource SecondaryIsEnabledTextBlockStyle}" />
                                     <HyperlinkButton
                                         x:Uid="GeneralPage_UpdateChannelLearnMore"
                                         Margin="0,2,0,0"