CmdPal: Update separator visuals in item details (#44620)

## Summary of the Pull Request

This PR updates separator in the details pane.

- If separator has a label
  - removes the line
  - makes top margin bigger
- keeps it based on BodyStrongTextBlockStyle to provide visual hierarchy
and anchor

- If separator has no label
  - makes line height only 1px 

<img width="399" height="810" alt="image"
src="https://github.com/user-attachments/assets/0ff0eb24-16f3-45a8-b23e-289809dc611d"
/>

<img width="403" height="802" alt="image"
src="https://github.com/user-attachments/assets/4171c954-9cd0-4622-8a95-8acd34ebe4c9"
/>


<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [ ] Closes: #xxx
<!-- - [ ] Closes: #yyy (add separate lines for additional resolved
issues) -->
- [ ] **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

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## 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/modules/cmdpal/Microsoft.CmdPal.UI/Helpers/BindTransformers.cs

@@ -18,6 +18,9 @@ internal static class BindTransformers
     public static Visibility EmptyOrWhitespaceToCollapsed(string? input)
         => string.IsNullOrWhiteSpace(input) ? Visibility.Collapsed : Visibility.Visible;
 
+    public static Visibility EmptyOrWhitespaceToVisible(string? input)
+        => string.IsNullOrWhiteSpace(input) ? Visibility.Visible : Visibility.Collapsed;
+
     public static Visibility VisibleWhenAny(bool value1, bool value2)
         => (value1 || value2) ? Visibility.Visible : Visibility.Collapsed;
 }

src/modules/cmdpal/Microsoft.CmdPal.UI/Pages/ShellPage.xaml

@@ -126,16 +126,16 @@
             </DataTemplate>
             <DataTemplate x:Key="DetailsSeparatorTemplate" x:DataType="coreViewModels:DetailsSeparatorViewModel">
                 <StackPanel Margin="0,8,8,0" Orientation="Vertical">
+                    <TextBlock
+                        Margin="0,8,0,0"
+                        Style="{StaticResource SeparatorKeyTextBlockStyle}"
+                        Text="{x:Bind Key, Mode=OneWay}"
+                        Visibility="{x:Bind help:BindTransformers.EmptyOrWhitespaceToCollapsed(Key), Mode=OneWay, FallbackValue=Collapsed}" />
                     <Border
                         Margin="0,0,0,0"
                         BorderBrush="{ThemeResource DividerStrokeColorDefaultBrush}"
-                        BorderThickness="0,0,0,2">
-                        <TextBlock
-                            Margin="0,0,0,0"
-                            Style="{StaticResource SeparatorKeyTextBlockStyle}"
-                            Text="{x:Bind Key, Mode=OneWay}"
-                            Visibility="{x:Bind help:BindTransformers.EmptyOrWhitespaceToCollapsed(Key), FallbackValue=Collapsed}" />
-                    </Border>
+                        BorderThickness="0,0,0,1"
+                        Visibility="{x:Bind help:BindTransformers.EmptyOrWhitespaceToVisible(Key), Mode=OneWay, FallbackValue=Collapsed}" />
                 </StackPanel>
             </DataTemplate>
             <DataTemplate x:Key="DetailsTagsTemplate" x:DataType="coreViewModels:DetailsTagsViewModel">