[UX] Settings - fix header position (#40160)

## Summary of the Pull Request
Due to a padding in the scrollviewer hosting the settingspage content,
the title drifts away upon resizing the settings window.
This PR fixes that issue by adding the (20px) padding to the maxwidth of
the header textblock.

Before:

![header](https://github.com/user-attachments/assets/6704a59d-843c-49a7-a109-e22e2cecccfe)

After:

![header2](https://github.com/user-attachments/assets/c9b0aa46-fcab-43ac-be87-ea423d810606)



## PR Checklist

- [x] **Closes:** #40159
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/settings-ui/Settings.UI/SettingsXAML/Controls/SettingsPageControl/SettingsPageControl.xaml

@@ -12,6 +12,7 @@
 
     <UserControl.Resources>
         <x:Double x:Key="PageMaxWidth">1000</x:Double>
+        <x:Double x:Key="PageHeaderMaxWidth">1020</x:Double>
         <tkconverters:DoubleToVisibilityConverter
             x:Name="doubleToVisibilityConverter"
             FalseValue="Collapsed"
@@ -26,7 +27,7 @@
         </Grid.RowDefinitions>
         <TextBlock
             x:Name="Header"
-            MaxWidth="{StaticResource PageMaxWidth}"
+            MaxWidth="{StaticResource PageHeaderMaxWidth}"
             HorizontalAlignment="Stretch"
             VerticalAlignment="Stretch"
             AutomationProperties.HeadingLevel="1"