CmdPal: Improve readability and a11y of sections (#44556)

## Summary of the Pull Request

This PR improves visuals and a11y of sections:
- Changes section title color from disabled to secondary text fill.
- Removes separator line if the text is present and indents
- Changes gallery grid subtitle color from tertiary to secondary text
fill.

## Pictures? Pictures!

<img width="850" height="718" alt="image"
src="https://github.com/user-attachments/assets/2e0bffa2-045f-48d9-bff5-dcc561395c6a"
/>

<img width="850" height="773" alt="image"
src="https://github.com/user-attachments/assets/69081472-7f4b-489a-b0e5-5778894fef97"
/>

<img width="850" height="773" alt="image"
src="https://github.com/user-attachments/assets/aa3f0e73-def8-45cb-9f9a-6e9d7e0e6137"
/>

<img width="850" height="773" alt="image"
src="https://github.com/user-attachments/assets/cb8f6bdf-9288-4b6e-b8b8-94f8c83e3ffc"
/>



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## 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/Controls/WrapPanelCustom/WrapPanel.cs

@@ -99,8 +99,6 @@ public sealed partial class WrapPanel : Panel
         set { SetValue(HorizontalSpacingProperty, value); }
     }
 
-    private bool IsSectionItem(UIElement element) => element is FrameworkElement fe && fe.DataContext is ListItemViewModel item && item.IsSectionOrSeparator;
-
     /// <summary>
     /// Identifies the <see cref="HorizontalSpacing"/> dependency property.
     /// </summary>
@@ -350,7 +348,7 @@ public sealed partial class WrapPanel : Panel
                 return;
             }
 
-            var isFullLine = IsSectionItem(child);
+            var isFullLine = GetIsFullLine(child);
             var desiredMeasure = new UvMeasure(Orientation, child.DesiredSize);
 
             if (isFullLine)