CmdPal: Add Acrylic backdrop to the context menu and tweak its style (#41136)

## Summary of the Pull Request

- Adds acrylic backdrop to the context menu
- Tweaks border of the context menu to match CmdPal aesthetics
- Acrylic backdrop requires ShouldConstrainToRootBounds="False",
otherwise the backdrop is not rendered

After:

Video:



https://github.com/user-attachments/assets/e32741a3-6bbb-4064-9e7f-84d7551b5164


Still:

<img width="1007" height="1313" alt="image"
src="https://github.com/user-attachments/assets/d6a7bd6a-d5d8-4674-9062-91f496f49f0c"
/>


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## PR Checklist

- [x] Closes: #41134
- [ ] **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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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI/Controls/CommandBar.xaml

@@ -41,14 +41,18 @@
                 <Style.Setters>
                     <Setter Property="Margin" Value="0" />
                     <Setter Property="Padding" Value="0" />
+                    <Setter Property="Background" Value="{ThemeResource DesktopAcrylicTransparentBrush}" />
+                    <Setter Property="BorderBrush" Value="{ThemeResource DividerStrokeColorDefaultBrush}" />
                 </Style.Setters>
             </Style>
 
+            <!--  Backdrop requires ShouldConstrainToRootBounds="False"  -->
             <Flyout
                 x:Name="ContextMenuFlyout"
                 FlyoutPresenterStyle="{StaticResource ContextMenuFlyoutStyle}"
                 Opened="ContextMenuFlyout_Opened"
-                ShouldConstrainToRootBounds="False">
+                ShouldConstrainToRootBounds="False"
+                SystemBackdrop="{ThemeResource AcrylicBackgroundFillColorDefaultBackdrop}">
                 <cpcontrols:ContextMenu x:Name="ContextControl" />
             </Flyout>